the anatomy of an adult chigger mite blankaartia acuscutellaris (walch)

The Anatomy of a n Adult Chigger Mite Blankacrrtia acuscu fellaris ( Walch) '

RODGER MITCHELL University of Florida, Gainesville, Florida

There is no complete description of the anatomy of an adult trombiculid and this is the second part of a study that attempts to meet the need for detailed anatomical information on this medically important group of organisms. Body musculature was fully described in the first section (Mitchell, '62) and certain aspects of other organ systems were briefly considered. In the following study the remaining systems - digestive organs, excretory organs and reproductive organs - are described and, since the body muscles provide the only criteria for defining locations, they are used for establishing locations. A precise definition of those muscles and the nomen- clature is given in the earlier paper.

Most of the structures considered here have been described by Henking (1882), Brown ('52) or Moss ('62) but none of these accounts is complete and it is inap- propriate to draw conclusions about trombiculid body architecture and organ functions without fairly complete data on all the structures of a species. Particular at- tention is given to the arrangement and relations of organs. Histological appear- ance or structure often seems to give a basis for some speculation about function but these interpretations are strictly limited because all the specimens were from a single rearing, and routinely cut at 8 mv. Hence, the discussions of function should be regarded with caution and looked upon as posing questions about, rather than ex- plaining, the functional and comparative anatomy of trombiculid mites.

MATERIALS AND METHODS

Specimens for this study were reared at the Institute for Medical Research at Kuala Lumpur (Malaya) by Mr. M. Nadchatram. Except where indicated below the descriptions that follow were based on the study

J. MOEPH., 114: 373-392.

of at least five complete serial sections of each sex. Adults about one month in age were fixed and stored a few weeks in Brazil (Baker, '59). Specimens were then run through a series of ethyl alcohols to 95% and transferred to 95% alcohol layered over a 5% solution of parlodion in methyl benzoate. When specimens passed to the bottom of the container the overlying fluids were removed and the methyl benzoate solution changed four times over a period of ten days.

These specimens were embedded by first removing the methyl benzoate in several changes of benzene and then gradually in- filtrating with paraffine-benzene at low temperatures (40-45°C) followed by a final imbedding of approximately two hours at 58°C.

Shrinkage was not measured. Any major body shrinkage would be shown by folding of the integument and in this case i t was not severe. There was probably marked shrinkage in the midgut which appeared to be drawn from the dorsum in all specimens. It is unlikely that much of the space between the mid-gut and body wall of sectioned material was haemocoele.

Area/volume estimates of a female (table 2) were taken from a series of sketches drawn with the aid of a split im- age drawing tube at 50 times magnifica- tion. Every third 8 mp section was drawn. Volume was obtained by weighing the cut- outs from the sketches and converting the weight to volume by a calculated constant. Surface area was estimated from direct measurement of the circumference of organs in sections. The main source of er- ror is shrinkage and the measurements are used only as general indicators of struc- tur a1 re1 ations . ~

1 This study was supported by grant no. 04403 from the National Institutes of Health.

373

3 74 RODGER MITCHELL

The mid-gut (figs. 1-2) The only opening of the mid-gut is from

the esophagus (fig. 2) which pumps food into the gut by means of a pharyngeal pump (Mitchell, ’62). It is unlikely that large particles can pass through the esophagus so the material entering the mid- gut must be liquid or liquified by action of oral gland secretions applied to prey tissues. Such feeding may bring very little indigestible matter into the mid-gut and may make a hind gut unnecessary. What- ever the explanation, there is no hind gut and, like other trombidiform mites, the residues left from digestion and absorption accumulate in the mid-gut which is a massive organ divided into a complex of lobes that fill most of the body.

The esophagus (fig. 2) opens into a small median connection between the two massive lateral lobes of the mid-gut. Ante- riorly and posteriorly the lateral mid-gut lobes are divided into dorsal and ventral lobes (figs. 1 and 2 ) . Except for the postero-dorsal lobe, the four terminal lobes are broadly continuous with the lateral lobes. An additional median gut lobe (fig. 1) originates from the right lateral lobe and may have a small posterior connection with the left lateral lobe.

The narrow connection of the postero- dorsal lobe with the lateral lobe lies just posterior to the level of dorso-ventral muscle 17 (fig. 1). The postero-dorsal lobe always has a great accumulation of black granules presumed to be the undigested food residue. These accumulations may completely fill the lumen of the postero- dorsal gut lobe and when the lobe is filled the passage from the lateral gut lobe seems to be occluded. The movement of materials into the postero-dorsal lobe must be accomplished by means of differential pressure applied to various gut regions by the body musculature. There does not seem to be a muscularis layer encasing the mid-gut.

Since the specimens considered here were from a mass culture and nothing was known of their prior feeding, it is impossible to add anything to the knowl- edge of trombidifonn gut cell histology established by Bader (’38). Within an individual the gut histology of a l l the lobes except the postero-dorsal lobe is constant.

There is little difference in the gut cell population of different individuals except for the accumulation of the presumed waste products.

The excretory organ (figs. 2-3) A simple, very thin-walled median excre-

tory tube extends nearly the entire length of the body (fig. 2). Anteriorly the tube is ‘T” shaped in cross section. Its two lateral extensions lie between the antero- dorsal and antero-ventral gut lobes and the ventral extension lies between the antero-ventral gut lobes. The three extensions are reduced posteriorly and the excretory tube assumes a dorso-ventrally flattened oval shape as it passes over the median connection of the gut. The median gut lobe displaces the excretory organ to the left in the posterior part of the body. Here the excretory organ is a flattened oval between the left lateral gut lobe and the median and right gut lobes (figs. 1, 2).

Immediately posterior to the genital organs the excretory tubule dilates to form an excretory chamber. A reduced posterior extension of the tubule extends posteriorly from the excretory chamber and lies between the postero-ventral gut lobes (figs. 2, 3). A small passage leads from the large excretory chamber to the excretory pore.

As indicated before (Mitchell, ’62), body muscles do not attach to the excretory tubule nor is there any evidence of a musculature investing the excretory tubule. Hence, pressure generated by the body musculature must evacuate the excretory tubule. Two muscles attached close to the excretory pore (excr. 2 and 5, fig. 2) would exert direct pressure on the excretory chamber. Other muscles near the excretory chamber, especially dorso-ventral muscles 16 and 17, could press against the chamber. The excretory muscles which lie against the venter (fig. 3) move or point the excretory pore but probably do not aid in evacuating the tubule.

A uniform layer of flattened epithelial cells lines the excretory organ. Most of this epithelium is in intimate contact with gut wall and only a small part of the other organs is in close contact with the excretory organ, thus, most dissolved materials transferred to the excretory organ may

ADULT CHIGGER ANATOMY 375

come from the mid-gut. The excretory product has not been critically examined in any prostigmatid mite and it seems unwise to assume the crystals in the excretory tubule to be guanine on the basis of their shape and size.

The excretory tubule, often called the hind-gut (Hughes, '59), has none of the histological and functional attributes of the hind-gut found in mites with a complete digestive tract. Instead, it resembles the excretory (or Malpighian) tubules of such mites. The origin of the excretory system has not been studied in the trombidiform mites, thus, embryonic homologies are not known. Hence, the use of a functional term, excretory tubule, is to be preferred over the term hind-gut, which is in common use, but may well be incor- rect as well as misleading.

Males possess paired accessory glands that empty through the excretory pore (fig. 3) . The large cells of the gland take a uniformly light stain and no secretions are present in the gland lumen. Females have no accessory structures. The function of these glands is unknown.

Oral glands ( f i g s . 2, 4-1 0) Oral glands of three of the higher

terrestrial trombidiforms are clearly described: Trombicula akamushi (Brumpt) (Obata, '54), T. alfreddugesi (Oudemans) (Brown, '52) and Allothrombium lerouxi Moss (Moss, '62). These and the present species show similar spatial arrangements of the glands and identical duct connections.

The tubular gland is easily distinguished by its shape but the other four glands are similar in general histology and variable in shape (fig. 7) and only their relative positions seem to show a pattern. The two laterally placed glands lie together. One is termed the dorsal gland and the other the lateral gland. Usually the remaining two glands are ventral to the above pair and these are termed the ventral and median glands. Thus, a nomencla- ture based on position works very well but should not, as emphasized by Moss ('62), be taken to reflect homologies. It is sig- nificant that gland location and duct connections are similar in all known forms. The duct pathways and connections are

complicated enough to indicate a functional correlation.

The ventral glands are always drained by a separate pair of ducts. A pair of common ducts drains the other glands and they always begin from the tubular gland (figs. 2, 6). Individual lateral ducts lead from the other four glands and join the common duct in the following sequence: lateral gland, dorsal gland and median gland. A tortuous path must be taken by the common duct in order to receive oral gland secretions in this order (fig. 7 ) and Moss ('62) argues that the arrangement in Allothrombium could allow mixture of the secretions of the tubular, lateral and dorsal gland while secretions of the median and ventral glands could be kept separate from each other as well as from those of the other three glands. There is no indication of a common storage chamber for

, tubular, lateral and dorsal gland secretions in Blankaartia but Moss's hypothesis could apply here. If it could be proven to apply, there would be a good functional reason for the arrangement of ducts and a sound basis for establishing homologies.

Except for the tubular gland, the shape of the various glands follows no pattern and there are no well established differ- ences in the histology of the glands. Glands of the three Trombiculidae species, 2'. akamushi, T . alfreddugesi and B . acuscutellaris, are packed tightly together as they are in some water mites (Bader, '38) but in the one known Trombidiidae (Allo- thrombium) they are widely separated.

Dorsal and Lateral Glands (figs. 5-7). No separation can be seen between the cells of these glands. Their lumina are separate and drained by separate ducts. Although the cells of both glands are of similar form, their size and depth of stain- ing differs slightly. The lateral gland cells stain less darkly.

Median and Ventral Glands (figs. 7, 8, 10). These glands also form a single mass. Median gland cells take a very light stain and are quite difficult to distinguish. If it were not for the presence of a duct and the distinct central lumen at the end of the duct, the gland could easily be over- looked. The ventral gland is easily distinguished and its large cells take a very heavy stain.

376 RODGER MITCHELL

Tubular Gland (figs. 4, 7, 9). In all the known Trombiculidae the tubular gland extends posterior to the median connection of the mid-gut and then ends shortly after it bends dorsally. In the one known Trombidiidae (Moss, '62) and at least one water-mite (Bader, '38) the tubular gland follows a long tortuous path and doubles back on itself. In Blanhaartia, as in Trom- bicula, the gland is a simple tube lined by cells that do not take a very heavy stain and are very difficult to discriminate. At the posterior end the gland broadens and the lumen anastomoses in that part

The arrangement of the mouthparts and their actions is discussed elsewhere (Mitchell, '62) but in figure 11 as well as in the text of that paper the common oral gland duct is erroneously identified as the trachea.

The genital system (plate 4) The several external sclerites of the gen-

ital field are similar in both sexes but none of these external elements are associated with any of the genital muscles or internal genital organs. They are either supportive, protective or sensory in function. The large external genital opening, which has three elongate sclerites associated with it, opens into a space that bears three oval sensory elements, the genital acetabula

(fig. 4).

(fig. 14), on its ventral wall. When the lips of this genital opening are spread, one can see the true gonopore which extends the full length of the genital field in the female (fig. 13) and is a very small opening in the male (fig. 11).

It is not pertinent to discuss the external genital sclerites here but they are used in plate 4 to show the locations of internal structures relative to the external genital field. Wherever possible the terms sug- gested by Feider ('59) are used for internal sclerites. A somewhat more precise definition of genital muscle attachments is possible and table 1 replaces the earlier descriptions of these muscles (table 9, Mitchell, '62). The numbering is unaltered and one muscle (gen. 7, fig. 17) is added.

Female reproductive system (figs. 2, 12-14 , table 1 )

The ovaries are usually a little less than 200 c1 long and lie ventral and lateral to the gut (fig. 2). Most of the mass of ovarian tissue is just lateral to the external genital field and may occupy as much as one-half the cross sectional area in a gravid individual. The two lateral masses are joined together by a narrow bridge of ovarian tissue posterior to genital muscle 6 (figs. 12, 14). The anterior halves of the paired ovaries are connected by a broad flat median oviduct that joins the

TABLE 1

Muscles 4 and 7 are f o u n d only in the m a l e At tachments of genital musc les . All but genital musc les 2 a n d 3 are illustrated o n plnte 2.

Insertion Origin

gen. 1 Anterior end, genital lips Postero-lateral projection postero-

gen. 2 Lateral margin, anterior genital Same as gen. 1

gen. 3 Lateral margin anterior genital Same as g e m 1

gen. 4 (d) Anterior surface, hypoapodeme Lateral surface, ejaculatory bulb

gen. 5 (d)

gen. 5 ( 0 ) Same as male Postero-lateral angle, oviduct

lateral angle, coxa IV

acetabulum

plate

Lateral margin, anterior genital Same as gen. 4 plate

gen. 6 (a) Lateral surface, ejaculatory Dorsal body wall bulb

gen. 6 ( 0 Postero-lateral angle, oviduct Dorsal body wall

gen. 7 (d) Hypoapodeme Operculum


meso-ventral surface of the ovary. The proximal portion of the oviduct is thin walled but the oviduct narrows and its walls thicken as it passes anteriorly from the ovaries. The oviduct extends 50 p or more anteriorly and then the oviduct turns ventrally and passes posteriorly to the vi- cinity of the gonopore (fig. 12). The cells lining the narrower portions of the oviduct may be secretory.

The oviduct ends blindly and it opens into the thin-walled genital chamber through a longitudinal slit-like opening on the floor of the oviduct (fig. 14). The margins of the opening are supported by sclerites and it appears the contraction of genital muscle 5 would draw the posterior portion of the oviduct down so that the longitudinal opening of the oviduct pro- trudes out of the gonopore. If this does occur, then the genital chamber would be everted by genital muscle 5 and retracted by genital muscle 6.

Paired spermathecae are present (figs. 13-14) and a short duct passes from the spermatheca to pores at the postero-dorsal angles of the genital chamber. These pores would be exposed if the genital chamber were everted and, if sperm packets were taken in between the walls of the genital chamber, the sperm could easily be forced into the spermatheca by the walls of the genital chamber.

A series of muscles lies on the wall of the oviduct and these are very difficult to interpret from sections. The posterior end of the oviduct is invested by a group of muscle fibers that seems to converge to attach with genital muscles 5 and 6 (fig. 13). This set of muscles may aid in ex- truding the egg. A second layer of muscles parallel, and seem to be associated with, the sclerites of the oviduct opening. These muscles extend from the postero- dorsal face of the oviduct just anterior to genital muscle 6 (fig. 13) to the anterior limit of the opening from the oviduct to the genital chamber. Neither the arrangement of fibers nor the function of the muscle is clear although this muscle may open the longitudinal slit in the oviduct floor.

Most of the ovigerous tissue is in the ventral and mesa1 part of the ovary with the more mature ova lying dorso-laterally.

As eggs mature they appear to pass toward the lateral cavity of the ovary and from there they move into the median oviduct.

The mature eggs in the oviduct seem to have a fully formed shell and these eggs pass through the oviduct singly. The eggs can easily move through the capacious thin-walled dorsal part of the ovary. No eggs were seen in the broad, thick-walled ventral portion of the oviduct so its modi- fications during passage of eggs are unknown. All portions of the oviduct lack an investing musculature so the force de- veloped by body muscles must be respon- sible for moving eggs through the oviduct. Fertilization probably occurs in the genital chamber near the pore from the spermatheca.

Male reproductive system (figs. 1 1 , 16, 17,

table 1) The large sack-like testes are about 450 I.I

in length and one-half to three-quarters of the organ is lined by a layer of columnar cells. These cells take a heavy stain, have the nucleus centrally placed, and show the general facies of secretory cells. The remaining testicular wall is occupied by gonial tissue which is not reduced with the diameter of the testes posteriorly; con- sequently, the posterior end of the testes is entirely gonial tissue. Dorso-ventral muscle 15 dorso-laterally and the origin of the vas deferens (fig. 11) mark the anterior limit of gonial tissue. An anterior testicular lobe passes around dorso-ventral muscle 15 and extends approximately 100 LI anteriorly (fig. 11) . Cuboidal cells line the lobe and the lumen is usually packed with sperm and secretory products. Sperm stored in the lobe could be forced back to the vas deferens opening by pressure de- veloped through contractions of dorso-ventral muscles 12 and 13 and genital muscle 1 which lie in the area of the anterior lobe. Since the lobe cannot be differentiated from the secretory portion of the testes proper, it is best termed an anterior testicular lobe even though it may function as a seminal vesicle. The secretory cells may secrete a seminal fluid which appears as lightly stained globules.

A thin-walled sheath encloses the testes at the level of dorso-ventral muscle 14 and

3 78 RODGER MITCHELL

it is continuous with the thick-walled vas deferens. Vasa deferentia from either side pass mesially and come to lie side by side on the mid-ventral line (fig. 11). The paired vasa deferentia extend about 100- 125 u anteriorly and then turn dorsally to reverse their course and pass posteriorly for 40 to 80 p before joining together to form a median duct that is broad in cross section and lined by very large columnar cells.

No distinct histological differentiation is apparent from the origin of the common duct to the gonopore but there are some structural elements that doubtless function to ejaculate the spermatophore. The role played by the various components is unknown but it is possible to postulate functions from a study of sections and these suggestions are at best a basis for more critical studies of spermatophore produc- tion and deposition.

The median duct produced by the fusion of the paired vasa deferentia is best termed an ejaculatory duct (fig. 11). In addition to the main gonoduct there is a broad ejaculatory bulb on the dorsal surface of the ejaculatory duct and a median accessory gland lying under the ejaculatory duct (figs. 11, 16, 17). These three structures form the ejaculatory complex.

Sheets of muscles surround the ejaculatory complex. The bulb is covered by a separate sheet of muscles that can cause the bulb to be evacuated (fig. 16). The ejaculatory duct and accessory gland are enclosed by a second sheet of muscle that extends from the base of the bulb to the apodema (fig. 16) and this muscle can constrict the duct and accessory gland.

In the region of the ejaculatory bulb the cells of the ejaculatory duct change from cuboidal secretory-like cells to flat epithelial cells and the epithelium forms sheets that are folded longitudinally and may serve as valves or guides (fig. 16). The valve-like opening of the ejaculatory bulb is surrounded ventrally and laterally by a similar valve-like structure of the ejaculatory duct (fig. 16). The bulb and the duct seem to have separate investing muscula- tures and it is conceivable that the contents of the bulb could be forced into the center of the ejaculatory duct. The.lining of the ejaculatory bulb does not appear to

be secretory and therefore its contents probably come through the vasa deferentia and ejaculatory duct. It would appear the bulb contents could be forced into the center of the duct and, if the bulb con- tained sperm and seminal fluid while the duct contents proved to be the supporting material of the spermatophore, then pro- duction of the spermatophore could be readily visualized. Since the bulb and duct are empty in all the material at hand, this must be regarded as a speculative inter- pretation unsupported by direct evidence.

The accessory gland is small in diameter and quite thin-walled (fig. 16). Its secretions, which are probably of a very limited volume, are passed into the terminal chamber of the ejaculatory complex. The functions of the secretions are not apparent.

Descriptions of the deposition of spermatophores (Lipvosky, et al., '57 and Mit- chell, '58) indicate that the external genitalia are moved and three muscles are involved in these movements. Action of genital muscle 5 (fig. 16) would force the entire ejaculatory complex outward through the genital valves. Genital muscle 7 (fig. 16) extends the pineal sclerite which supports, at its tip, the gonopore and genital muscle 4 (fig. 16) retracts the pineal sclerite. Retraction of the entire complex results from the contraction of genital muscle 6 (fig. 16).

Nervous system (figs. 17, 18) The fused, undifferentiated central nerv-

ous mass is difficult to designate by an accurate morphological term and it is most convenient to call it a brain. A slight constriction separates the dorsal, or supra- esophageal, region from the ventral, or subesophageal region. As in other mites (Hughes, '59) the nerve trunks arising above the level of the esophagus are associated with feeding and sense organs. Al- though it has been assumed that these trunks are sensory (Hughes, '59), there is no real evidence for that claim. The trunks originating below the level of the esophagus innervate legs and other organs of the posterior body region.

A thin membrane surrounds the brain and the outer part of the brain consists of a layer of cell bodies that encircle a central neuropile. There is no marked separation


of regions apparent from a general exam- ination of the neuropile.

Moss ('62) carefully summarized the descriptions of nerve trunks in trombidiform mites and he usually named trunks according to the organ innervated. That will be followed here.

Three trunks arise dorsal to the level of the esophagus. The most dorsal nerve passes dorsally to sense organs associated with the scutum (fig. 18). One branch passes to the sensillary area of the scutum and another branch to the ocellar region (figs. 17, 18). Another trunk originates just ventral to the preceding nerve and parallels it briefly (fig. 18) but its course then reverses and it passes anteriorly to the chelicera. This is a very fine nerve designated as the second chelic- era1 nerve (Chelicera 2, figs. 17, 18). One of the nerves of this pair gives off a branch which passes to the rostral region. The rostral nerve came from the right trunk in the two slides in which the origin could be positively traced. In other species the rostral nerve originates as an unpaired nerve from the brain mass (Moss, '62).

The most ventral nerve from the dorsal portion of the brain arises just lateral and dorsal to the esophageal passage and this nerve serves the two appendages of the mouthparts (Chiliceral and Palp, figs. 17, 18).

A main trunk passes to each of the legs and each leg nerve divides to form a large main nerve and a finer collateral, a pattern known in three related mites (Moss, '62). The two pairs of nerves that originate on the posterior face of the brain pass to the genital region.

A fine branch arises, just antero-dorsal to leg 3 nerve (nerve X, fig. 17) and this nerve is comparable in origin to the nerve termed the lateral idiosomal nerve by Moss ( '62). Another fine branch (nerve Y, fig. 17) originates between leg 4 nerve and the nerves to the genital regions. These two fine nerves follow characteristic courses (fig. 17) but their innervation could not be determined and it seems unwise to name them at this time.

Body architecture The most conspicuous peculiarities of

trombidiform mites in contrast to other ar-

Dorsal Nerves.

Ventral Nerves.

thropods are ( 1 ) the lack of any organs for the transport of either gases or aqueous solutions and (2) an immense blind lobate mid-gut that occupies most of the body. Both features are involved in the transport of nutrients and possible metabolic by- products. In the absence of circulatory devices such molecules could be distributed if the fluid of the very reduced haemocoele were moved about by incidental body movements or by the body musculature. The latter muscles do not seem suitable for this task and the arrangement of organs is such that extensive haemocoeliac movement need not be postulated.

The body of this mite is oval and, except for the oral glands in the anterior of the body, there seem to be no serious restric- tions as to the arrangement of organs in the body cavity. Even the locomotor musculature is strictly limited to a thin layer on the venter and does not extend into the body cavity. The vast majority of the muscles that pass through the body cavity are very small bundles of muscles that main- tain body shape and turgor.

Hence, it appears that the placement of internal organs could be determined by the need for transportation of molecules in solution. An appreciation of the nature of the circulatory requirements of the mite can be obtained by examining the surface area and volume relations of the organs and the position of organs relative to each other and relative to the integument (the site of gas exchange), the mid-gut (the source of nutrients), and the excretory organ (where metabolic wastes are removed).

Such considerations must be based on clear information about geometry and it is very unsatisfactory to employ routine serial sections as a source of quantitative data on surface area and volume. But there are no estimates that give even the order of magnitude of surface area/volume relations in a mite so it is reasonable to obtain some crude estimates rather than continue the neglect of this aspect of mite morphology. The data of table 2 are very limited but they do show relations worthy of more extensive study.

The huge mid-gut need not be lobate (fig. 1) in order to accommodate the body muscles. All adjacent lobes have muscles

380 RODGER MITCHELL

TABLE 2

undeveloped eggs in the ovary was measured and the procedure is given under Materials and Methods

Surface Area/Volume relations of a female Blankaartia ascuscutellaris. One specimen with

Surface area Surface

Body 0.3013 mm3 2.5716 m m z 8.53

Mid-gut 0.1614 mm3 2.9967 mmz 18.57

Excretory organ 0.0089 mms 0.42260 mm* 47.48

(external) area/volume Volume

passing between them but these muscles could pass through a narrow cylindrical passage as is the case in some body muscles (d.v. 14, right hand excr. 2, fig. 1) . The tendency for the gut to be divided into lobes by the body muscles rather than to surround and encase the muscles is re- sponsible for the outer mid-gut surface area being 1.16 that of the body surface. The large surface area/volume ratio of 18.6 is a measure of the potential exchange of materials between the mid-gut lumen and tissues adjacent to the surface of the gut. This means that the lobate gut must be extremely effective in losing absorbed nutrient to adjacent tissues.

About 65% of the excretory epithelium is in direct contact with or immediately adjacent to a gut lobe and the ratio of the area of gut-excretory organ contact to total excretory organ volume is 31.0. The ratio of the remaining excretory epithelium to total excretory organ volume is 16.5. The only active organs, other than mid-gut, that lie close to the excretory organ are the dorsal muscles in the anterior body region (fig. 1) and this area represents a fair fraction of the freely exposed excretory organ epithelium not adjacent to mid-gut. If all the cellularly uniform epithelium of the excretory organ functions similarly, then much of the material excreted by the excretory organ must diffuse from the mid-gut.

Alternatively the exposed excretory epithelium might absorb fluids indiscrimi- nately from the haemocoele and selectively return water and other solutes to the body by way of apposed gut-excretory organ surfaces. The process could, of course, work in either direction. This is a more complicated mechanism than can be readily supported from anatomical facts and

the assumption that all the excretory epithelium acts in the same way is the more parsimonious postulate.

The remaining organs, oral glands, reproductive organs and brain, are ventral to the mid-gut and, as a rule, the dorsal and lateral surfaces of these organs lie against or close to the mid-gut. The venter of the organs is close to the integument (fig. 2). Thus, gas exchange of these organs is toward the venter while exchange of other molecules is toward the mid-gut.

All the active organs other than gut tend to be as far anterior as possible. Only the reproductive organs and excretory organ lie with the mid-gut in the posterior body region. Reproductive organs lie between the gonopore and the constriction between the anterior and posterior body regions. That constriction (fig. 1) is a physical barrier and the gonads extend to that limit. The posterior quarter of the body is mid-gut and a very small posterior extension of the excretory organ (account- ing for less than 5% of the total excretory organ surface area). Thus, the posterior part of the body is little more than a cavity for the storage of wastes.

Most aspects of the structure and arrangement of organs in B2anhaartia can be easily explained if the mid-gut is viewed as modified for the transport of materials to all other organs of the body and, since the mid-gut lies between these organs and the excretory organ, it is likely that transport of metabolic by-products is included among the mid-gut functions. The localization of digestive residues in one gut lobe could only occur if the movement of materials in the mid-gut lumen were under some control. Any controlled movement of the gut contents would increase the circulatory efficiency of the organ.

ADULT CHIGGER ANATOMY 38 1

This indication of circulation of mid-gut contents, as well as the arrangement of organs in the mite suggests a gastro-vascular function for the mid-gut. The incidental circulation of fluids in the haemocoele may provide an adequate exchange of materials between the organs, but that hypothesis is less attractive at present because it provides no explanation for the peculiarities of body form and architecture.

SUMMARY The compact body of this mite is volu-

metrically more than one-half mid-gut and this lobate organ lacks any posterior connection for removal of wastes.

The posterior third of the body is occupied by mid-gut lobes that store digestive wastes and all organ systems lie anterior to this storage region.

Crystalline metabolic wastes are elimi- nated by way of an excretory tubule. Usu- ally the mid-gut lies between the tissues of high activity (most muscles, the nervous system, and the reproductive system) and the excretory tubule, hence it appears that the mid-gut may assume the functions of a gastro-vascular cavity.

With the exception of the oral glands, most active organs lie against the integu-

ment and can therefore exchange gases directly through the integument.

LITERATURE CITED Bader, C. 1938 Beitrag zur Kenntnis der Verdau-

ungsvorgange bei Hydracarinen. Rev. Suisse

Baker, J. R. 1959 Principles of Biological Micro- technique. London: Methuen. 357 pp.

Brown, J. R. C. 1952 The feeding organs of the adult of the common “chigger.” J. Morph., 91: 15-52.

Peider, Z. 1959 Etude des Charact&res Sexuels chez les Trombidoidea. Acarologia, 1 : 5-5.

Henking, H. 1882 Beitrage zur Anatomie, Entwi- cklungsgeschichte und Biologie von Trombt dium fuliginosum. Herm. Zeitschr. wiss. Zool.,

Hughes, T. E. 1959 Mites or the Acari. London: Athlone Press. 7 + 225 pp.

Lipovsky, L. J., G. W. Byers and E. H. Kardos 1957 Spermatophores - the mode of insemi- nation of chiggers (Acarina: Trombiculidae). J. Parasit., 43: 256-262.

Mitchell, R. 1958 SFerm transfer in the water mite Hydryphantes ruber Geer. Amer. Mid. Nat., 60: 156-158.

1962 The musculature of a trombiculid mite, Blanhaartia acuscutellaris (Walch). Ann. Ent. SOC. Amer., 55: 106-119.

Moss, W. W. 1962 Studies on the morphology of the trombidiid mite Allothrombium lerouxi Moss (Acari). Acarologia, 4: 313-345.

Obata, Y. 1954 Anatomical studies on Trombi- cula akamushi (Brumpt), the vector of tsutsu- gamushi disease. Sanitary Zool., 5: 111-145. ( In Japanese. )

ZOO^., 45: 721-806.

37: 553-663, pls. 34-36.

PLATE 1 EXPLANATION OF FIGURE

1 Dorsal aspect, mid-gut and body musculature of B . acuscutellaris.

Abbreviations

dors., dorsal muscle d.v., dorso-ventral muscle excr., excretory muscle g.L, gut lobe

382

ADULT CHIGGER ANATOMY Rodger Mitchell

d.v 3

dx 7, \ dors. 1

d.v \ dors.2 /

PLATE 1

4

1 dors. 6

303

PLATE 2

EXPLANATION OF FIGURES

2 Left hand aspect of a female B. acuscutellaris showing a section cut just left of the mid-line. Gut form, dorso-ventral muscles and excretory system are labeled. Oral glands and reproductive systems are indicated in proper position and are comparable to figures 7 and 14, respectively.

Dorsal aspect, excretory pore and associated structures of the male. 3

A bbreviatwns

excr., excretory gen., genital d.v., dorso-ventral muscle

384


PLATE 2

lateral b1. meJkn. g. 1. postero-dorsal 8; 1.

antero-dorsal g. 1. I

antero-ventral 1. / \ e L - L e r / ’ \I excr. 2 excr. 5

I connection 8: 1.

excr. t ubde

I

385

PLATE 3

EXPLANATION O F FIGURES

Oral Glands of B. acuscutellaris

The heavy arrows indicate the level of the cross sections illustrating gland histology. The upper scale applies to all cross sections (figs. 4-6 and 8-9), and the lower scale to figure 7.

4

5 Cross section, lateral gland.

6 Cross section, dorsal gland.

7

8 Cross section, ventral gland.

9

Cross section, terminal part of the tubular gland.

Right hand aspect, oral gland complex.

Cross section, proximal portion of the tubular gland.

10 Cross section, median gland.

386


PLATE 3

387

PLATE 4


Genital system of B . acuscutellaris

In order to show the position of internal organs relative to external genitalia, figures 11 and 1 2 are positioned relative to a rough sketch of external genitalia that is placed between those figures.

11

12

13

14

15

16

Ventral aspect male genital system (right testis is not shown).

Dorsal aspect female genital system (right ovary is not shown).

Right hand aspect of the structures adjacent to the gonopore.

Sagittal section of the structures shown in figure 13.

Sagittal section of the ejaculatory complex of the male.

Right hand aspect of the ejaculatory complex of the male.

Genital muscles are completely described in table 1.

Abbreviations

acces., accessory ant., anterior d.v., dorso-ventral muscle e j ac., ejaculatory

388


PLATE 4

a

V~IS deferens

operculum

389

PLATE 5


Nervous System of B. acuscutellaris

17

18

Dorsal aspect of the brain and major nerve trunks.

Right hand aspect of the brain and major nerve trunks.

390


PLATE 5

T 05

nerve

nerve

3

24

esopha us 8

391

the anatomy of an adult chigger mite blankaartia acuscutellaris (walch)

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