EXTERNAL ANATOMY

1. General

The general relation of the parts to one another is shown in figure 1. It will be seen that the head, thorax and abdomen are clearly separated from one another; that the thorax shows little external sign of segmentation; only seven abdominal segments are distinct. The insect is flattened dorsiventrally. There is one thoracic spiracle, which probably appertains to the prothorax. The leg of Pediculus, which is characteristic of that of Anoplura in general, is shown in figure 2; all parts of the leg are short and strong. The tibia is short and bears a blunt "thumb" on the inner side close to the apex; the thumb terminates in spines and hairs. The one-jointed tarsus is short and carries a single large claw, rough along its concavity. Between the thumb and the claw the louse grasps its host's hair or clothing. The three pairs of legs are similar in structure. Although it appears, at a cursory examination, that the abdomen (figure 1) consists of seven segments, there are slight indications in Pediculus, and clearer evidence in certain other Anoplura, that two segments precede the first evident segment, which bears the first pair of abdominal spiracles: this first evident segment, with the first pair of spiracles, is probably the third segment. There are six pairs of abdominal spiracles, carried on segments 3 to 8. All spiracles (including that on the thorax) possess a closing apparatus, which appears to work by compressing or buckling the trachea a short distance below the surface of the body (figure 3). It seems highly probable that the louse uses this in order to reduce the loss of water from within its respiratory system. For anatomy of egg, see page 24; for larva, page 28. For fuller accounts of both external and internal anatomy, see Hase (I93I), Patton and Cragg (I9I3); many points are figured by Keilin and Nuttall (I930).

FIG. 1. Pediculus humanus corporis, male, from above. a anus, an, antenna, d, claw, f, femur, p, pleural plates, s, spiracle; so, sexual orifice; t, titia: ta, tarsus. (After Keilin and Nuttall.)

2. Sexual Differences

The adult males and females are readily distinguished. The male is generally the smaller, but there is some overlap in the measurements of total length (see page 12). There are dark transverse bands on the dorsum of the abdomen of the male (figure 1), most evident in the highly pigmented specimens. The abdomen of the female is relatively the broader.

There are also considerable differences in the legs (figure 2), the anterior leg of the male being much the stouter, and provided with a larger tibial thumb and tarsal claw; with this the posterior leg of the female is grasped in copulation. The posterior end of the male's abdomen is rounded. The ventral surface curving upwards so as to bring the anus and sexual orifice to the upper surface. In the sexual orifice (SO) one may sometimes distinguish the dark pointed tip of the dilator (figure 1). In the female the abdomen terminates in two larger posterior lobes, which give it a bi- lobed appearance. In ventral view (figure 4) one observes the pair of gonopods on either side of the vaginal orifice; also a shield-shaped pigmented area, easily visible to the naked eye.

FIG. 2. Pediculus humanus. A, terminal part of anterior leg of male; B, of female; cl, claw; ta, tarsus; t, tibia. (After Keilin and Nuttall.)

FIG. 5. Pediculus humanus, male forcibly separated from female during copulation promptly killed: the abdomen is viewed from behind. ae, aedeagus; s, spiracle; v, vesica; 6-9 abdominal segments. (After Nuttall.)

It is only by dissection and observation of specimens in the act of pairing that the very complex copulatory organs of the male may be understood. The male approaches the female from behind and walks forward beneath her abdomen, seizing the base of her posterior femora with his first legs. The insects then assume a vertical position, head downward, the male being carried completely clear of the surface on which they have been walking. At this moment the male extrudes the dilator, which is hooked into the vagina, into which it penetrates deeply. It is then withdrawn and flexed out of the way, and at this instant the penis or aedeagus is passed into the vagina. A great part of the acdeagus (figure 5) consists of a thin-walled chitinous sac (the vesica), which fills the vagina, teeth on the walls of vagina and vesica anchoring the animals together. It is only after this that the ejaculatory part of the organ performs its function (Nuttall, I917a).

In the study of typhus (page 78) it is frequently necessary to discover the exact position of the anus in order to infect lice by injection. In the male this orifice (a) is on the upper surface immediately in front of the transverse sexual orifice: it lies in a minute pit (figure 1). In the female it is at the extreme posterior end of the body, between the posterior lobes and dorsal to the gonopods and vagina (figure 6).

FIG. 6. Pediculus humanus, longitudinal section, almost in median plane, of posterior part of female abdomen. a, anus; pl, shield-shaped plate; r, recturn; so, sexual orifice; 6-8, abdominal segments. (After Nuttall.)

3. Sense Organs

The structure and function (page 386) of the sense organs has been investigated by Wigglesworth (1941). The antenna carries sensillae of three types.

FIG. 7. A. dorsal view of left antenna. a, peg organs with rounded tips, b, peg organs with sharp tips; c, tuft organs; d, tactile hairs; e, scolopidial organs. B, detail of tactile hair, f, trichogen cell, tormogen cell; h, sense cell. C, detail of tuft organ. D, detail of peg organs. i, sense cells; k, nerve. (Wigglesworth, 1941.)

(a) Peg organs. These are limited to the fifth joint of the antenna, where there are nine or ten. The projecting peg is sharp in some organs, blunt in others; it is thin walled, and the cavity contains a process or filament which can be traced down to a group of sense cells (figure 7, D).

(b) Tuft organs. These are confined to the fourth and fifth joints of the antenna. Each consists of a minute cone lying in a shallow depression: at the apex of the cone is a tuft of four minute hairs, the base apparently connected by a rod or filament to a group of sense cells (figure 7, C).

(c) Tactile hairs. These are distributed in small numbers on all parts of the antenna. They are of a type very common in insects, and consist of a slender articulated bristle, connected with a sense cell.

(d) Scolopidial organs. Inside the second segment of the antenna are certain sense organs which are believed to be proprioceptive, i.e. to record movements and strains, and give the insect information as to the position of the antenna.

No difference has been detected between the antenna! sense organs of the male and female louse.

The eye is a single ocellus, on the lateral aspect of the head. The structure is shown in (figure 8).

FIG. 8. Horizontal section through eye.
h, corneal lens; i, corneagenous cells; k, rhabdoms; l, pigment; m, nuclei of retinal cells. (Wigglesworth. 1941.)

Numerous other sense organs occur, tactile hairs on many parts of the body, and proprioceptive organs of several types which give the insect information as to the position of the successive parts of its limbs.

4. Head and Body Lice

Before leaving the subject of external anatomy it would be well to consider the differences between Pediculus from different parts of the human body.

Lice (Pediculus) found on the human head usually differ slightly from those found on the body and clothes, and they were regarded as distinct species and named capitis and corporis by de Geer before the end of the eighteenth century. That view continued to be accepted for more than IOO years, most authors regarding the insects as distinct species, though the points of difference were admittedly difficult to define. But in 1919 Keilin and Nuttall published the results of a careful examination of a large collection of specimens from many parts of the world, and concluded that the head louse and body louse should not be regarded as separate species: subsequent work (Ferris, 1935) supports this view. It therefore seems best to refer to the head and body louse as physiological or biological "races", belonging to one species, Pediculus humanus (Linn, 1758); the appropriate Latin names are P. humanus capitis (de Geer, 1778) (the head louse) and P. h. corporis (de Geer, I778)(the body louse). The name vestimenti (Nitzsch, 1818) for the body louse is a synonym of corporis and should not be used.

The best anatomical basis of distinction between these races is provided by the antenna, which is relatively shorter and broader in capitis. In particular, the third joint of the antenna is relatively broader in that variety (figure 9), though recent work by Ferris (1935) has shown that the distinction does not invariably hold good: no large body of measurements is available. A number of other points of difference have been shown to be variable and unreliable. It is generally true that head lice are smaller than body lice, as the following approximate figures for mean total length (mm.) show:

 

But the total length of this insect is an unsatisfactory character, for it depends to some extent on whether it has recently fed and on the development of the ovaries in the female; in addition, methods of killing and mounting will affect the measurement.

FIG. 9. A, antenna of female Pediculus humanus corporis; B, of female Pediculus humanus capitis; both from England. (After Ferris.)

Head lice are generally more deeply pigmented, but the depth of the colour of Pediculus depends on the colour of the background on which it was reared, so that the difference is not constant and probably not inherited; moreover, it frequently happens that different parts of the body of an individual louse are pigmented to different degrees. Head lice may generally be recognized because the indentations between successive abdominal segments are more clearly marked than they are in body lice; this is due to differences in the chitinous pleural plate (p), also known as the parategral plate, or laterosclerite, which covers that part of the segment (figure 1). In general, the legs of capitis, are said to be relatively shorter than those of corporis. Other points of difference have been recorded, but they are inconstant and intermediates are frequently met with, both in wild material and in insects of known history reared in the laboratory. At the same that there is rarely any difficulty in identifying a head or body louse on general appearance. In culture the differenecs do not, in our experience, tend to disappear.

The subject is fully dealt with by Ferris (1935); this author's wide knowledge of the structure and classification of all types of Anoplura gives particular importance to his work. He regards head and body louse as the extremes of a continuous series, stating that every intergradation exists and even doubting if they are worth recognition as races.

It is not known whether reliance can be placed on differences in size and shape which are stated to be observable in the eggs of head and body lice (see figure 17).

It is a matter of importance that the various points of difference should be examined biometrically, so as to assess the extent to which they vary and overlap, and define the morphological points on which individual lice, or strains, may be identified. At present it is often accepted that lice from the head are "head lice" (and so they are in a certain sense). Biometrical work of this type would permit us to study such things as the migration of head lice to the body, or to other heads; it might be the basis of study on the cytology and genetics of the head and body louse: it would also give precision to our views on the head louse as a vector of typhus.

On the biological side the differences are more easily perceived. In general, lice from the human head attach their eggs to hair (page 24). They are also more active at lower temperatures than lice from the body, perhaps because the temperature at which they normally live on the exposed scalp is lower than that on the clothed parts of the trunk. Lice derived from the clothes od the surface of the body, on the other hand, attach their eggs to the inner side of the clothes. But the distinction living on the the head and the body is not very sharp, and there are records of lice with the anatomical characters of capitis establishing themselves on the body and occurring both there and on the head; Keilin and Nuttall (1919) examined several large collections of lice taken from the surface of the body, but appertaining to both races. It seems then that specimens which would be referred to as capitis frequently occur on the surface of the body. The occurrence on the head of specimens which appear to be corporis is less frequent. We may conclude, then, that the head louse and the body louse may occupy the same territory and that they have every opportunity of interbreeding.

Several authors have reared capitis in captivity, in boxes applied to the skin of the body (for method, see page 142), and have recorded that after a number of generations the specimens become intermediate, or assume the anatomical characters (size, length of leg, etc.) of corporis. This indicates that the anatomical differences are not constitutional or genetical, but capable of being altered by environmental factors. It should, however, be said that the experience of my colleague, J. R. Busvine, is different: after many generations of being reared under identical conditions the head and body lice remain distinct in general appearance.

In the eight months from September 1940 during which London was bombed, a large part of the population was living crowded in basements, shelters and so forth. During the first part of the period general hygienic conditions were very bad, and people were much crowded, large numbers often sleeping in close contact. Head lice were certainly common throughout the period, at least in the children, but there was no evidence that body lice occurred, except in the destitute, though a careful look-out was kept. The evidence from this large experience does not suggest the transformation of head lice into body lice, but supports the view that the two are biologically distinct.

Hybrids between the head and body louse may readily be raised in the laboratory; these hybrids are fertile, at least to the third generation, beyond which they have not been studied. In some families of hybrids intersexes are produced, that is to say, individuals in which some of the external or internal characters appertain to the one sex and some to the other. The fact that intersexes have also been found in wild populations of lice suggests that hybridization between the races also occurs in nature. The production of intersexes in hybridization shows that there is a considerable genetical difference between the two ancestors; some lack of harmony in the normal division of chromosomes occurs in hybridization, so that the factors which generally determine sex and produce either a male or female occasionally produce an intersex. It would be a valuable contribution to our understanding of the matter if a cytological study could be made of the chromosomes of parent races and hybrid (especially intersex) offspring.

We have very little information on the occurrence of intersexes in the absence of hybridization of the two races. Musgrave (1944) has recently found intersexes in a laboratory strain, which had been derived from garments, and was regarded as corporis. Roy and Ghosh (1944a) report examining 8,700 adult lice from heads of refugees entering India from Burma; no intersexes were found.

From the facts set out it seems clear that though the head louse and body louse are not identical, they are much more closely related and difficult to distinguish than "species" generally are. Probably, therefore, we should be correct in thinking of them as entities which have not yet evolved very far from one another; indeed, they might be called "species in the making". Inasmuch as the differences between them seem greater in biology than in anatomy they should be referred to as biological or physiological races; this is consistent with general zoological practice.

5. Lice from Different Human Races

It has frequently been asserted that lice from different races of man differ from one another in colour, those from dark races tending to blackness, those from pure Nordics tending to be blond. The work of Nuttall (1919) showed that the colour of a louse was greatly modified by that of its surroundings early in life. Differences might therefore arise between individuals of a single family, reared on different men, and it is probable enough that lice from a man with black skin and hair might be darker than those from a fair host. It seems, then, that most of the differences said to occur between the lice of Chinese, Europeans, American Indians and other human races are slight and inconstant, and due to colour and other environmental factors. It has been stated that lice from Africans (heads and bodies) differ from those of other human races in a number of anatomical particulars. Ferris (1935) goes fully into the matter and concludes that the characters are variable, and that they show intergradation with races capitis and corporis.

No human race (so far as is known) is without lice, or immune to them.

INTERNAL ANATOMY

1. Mouth Parts

FIG. 10. Pediculus humanus, longitudinal section of head.
bf, buccal funuel; cbp, cibarial pump; dp, dilator muscles of pharynx; h, haustellum; o, oesophagus; p, pharynx; sd, salivary ducts; sp, sphincters; ss, stylet sac. (schematic, modified from Peacock 1918, and Sikora, 1916, central nervous system omitted.)

The mouth parts are invaginated into the head except when the insect is feeding, and it is only by dissection and the study of serial sections that their relations to one another can be understood. The essential facts are shown in figure 10. In the front of the head is a small tube, the haustellum, which is believed to be formed from the labium; it is soft, eversible and armed with teeth. In figures 10 and figure 14 it is shown in the retracted position, but in figure figure 11 the haustellum is everted and the teeth rotated outwards; it will be seen that the opening is terminal and continued as a ventral cleft, the prestomum. That part of the alimentary canal which lies inside the head is separated into four regions (figure 10). The most anterior part is the relatively rigid buccal funnel. Behind are the cibarial pump, and the pharynx, each independently capable of dilation by dorsal muscles-arising from the head capsule and divided from one another by a constriction; between them, not shown in figure 10, is the frontal ganglion, an important landmark to the morphologist; the pharynx has strong sphincter muscles. Behind the posterior part of the pharynx comes the commencement of the oesophagus, which is narrow and not muscular.

FIG. 11. Pediculus humanus, everted haustellum seen from below, showing teeth,and part of shafts of stylets. (x1000.) (After Sikora.)

In the floor of the buccal funnel is the opening of the stylet sac, which is blind. Within this sac are three stylets lying one above the other (figure 12). It appears, from the study of the development of the embryo that the dorsal and ventral stylets are formed from a part of the rudiment of the second maxillae, which unite to form the labium in most insects. Early in embryology one may recognize paired mandibles and first maxillae, which disappear later (Scholzel,1937); other views have been put forward (e.g. Fernando, 1933).

FIG. 12. Pediculus humanus, stylets as seen from left side (the forked structures at the base being those of the left side only).
ds, dorsal stylet, hyp, part of hypopharynx; vs, ventral stylet, A, tip of ventral stylet (x1250) showing elaborate piercing end and groove in which the hypopharynx lies. (Modified from Peacock and from Vogel.)

FIG. 13. Pediculus humanus, head from below to show ventral styles and protractor muscles.
an, antenna; ap, anterior part of pharynx; bf, buccal funnel pbf, protractor of buccal funnel rbf,retractor of buccal funnel; ps,protractor of stylet; vs,ventral stylet. (x195) (After Sikora.)

The dorsal and ventral stylets are very similar in shape, forked behind but united in front and sharply toothed at the tip (figure 12, A). Immediately below the upper stylet is a fine tube, formed from a part of the hypopharynx and pierced by the common salivary duct: it is the "intermediate stylet" of Snodgrass (1944). It is continuous with the common salivary duct, and is tubular. There is some doubt as to whether the intermediate stylet is fused at its base with the dorsal stylet. The three stylets (i.e. dorsal, ventral and intermediate, or hypopharynx) are capable of being brought forward through the prestomum by protractor muscles (figure 13). When not in use they lie entirely within the head. The relationship of the stylets, and something of their function, is made clear in (figure 14), which shows them in transverse section. The dorsal stylet, made apparently from two united halves rolled upwards, forms the food canal. Piercing the skin is probably carried out by the relatively robust ventral stylet: it embraces the other stylets and tends to hold them together as a fascicle. The very fine intermediate stylet, or hypopharynx is seen to be penetrated by a canal, which we believe to be continuous with the common salivary duct.

FIG. 14. Pediculus humanus, transverse section of haustellurn in the retracted position.
ds, dorsal stylet; hyp, hypopharynx; th, teeth of eversible part of haustellum; vs, ventral stylet, (x about 1000.)(After Vogel.)

The description and interpretation of the mouth parts of the louse is a matter on which authorities are not unanimous. The above is derived mainly from Peacock (1918), Sikora (1916) and Vogel (1921). Snodgrass (1944), our principal authority on the comparative anatomy of insects, has recently discussed the matter; the interpretation I have given does not differ from his except in points of nomenclature.

In feeding, the head is depressed and the soft chitinous haustellum thrust out so that the everted teeth anchor the insect's head to the epidermis of its host. The protractor muscles thrust the stylets forward through the opening in the haustellum and move them in and out so that they pierce the skin and lacerate the tissues beneath it; the whole buccal funnel is moved forward at the same time. After the stylets have pierced the skin, saliva is presumably delivered down the hypopharynx by way of the common salivary duct. The mechanism by which blood is taken into the alimentary canal is not certainly known; but it seems probable that the soft eversible haustellum fits closely against the skin and that the capillary blood pressure is sufficient to cause the blood to flow from the capillaries into the mouth along the trough formed by the upper styles (see unpublished observations of Wigglesworth, page 37). The contraction of the dorsal muscles of the cibarial pump would also give some suction; it seems probable that when the insect is taking blood, the constriction between the cibarial pump and the pharynx acts as a valve and is closed, so as to prevent blood which has already been swallowed from being sucked forward towards the mouth. When the dorsal wall of the pharynx is relaxed the blood would be able to flow to the oesophagus, towards which it might be propelled by the sphincters in the wall of the pharynx. This explanation of the mechanism is supported by observations made on the living insect while in the act of sucking; this may most conveniently be done upon a larval louse, the head of which is much more transparent than that of an adult. It can then be seen that while the louse is sucking blood there is a rhythmical filling and emptying of the cibarial pump several times per second.

2. Alimentary Canal

FIG. 15. Pediculus humanus, internal anatomy of male seen from above.
a, anus; cns, ventral part of central nervous system; d, dilator; ht, heart; mg, midgut; mt, Malpighian tube; my, mycetome; r, rectum; sd and sg, salivary duct and glands; ts, testis, (After Kahn and Nuttall.)

The general disposition of the alimentary canal, which is about twice the length of the insect, is shown in figure 15. It has already been explained (page 16) that the foregut consists of four parts; the last of these, the oesophagus, which is straight and narrow, enters the midgut in the thorax. The anterior part of the midgut is lobed, forming two gastric caeca; it is large and distensible, and occupies a great part of the abdomen. After a meal it contains much blood, and its active peristaltic movements may be seen through the integument of the living insect. There is no peritrophic membrane, and no crop. The mycetome, a conspicuous round object on the ventral wall of the midgut (my, figure 15, is discussed below (page 21). The posterior part of the midgut is narrow and apparently less distensible. At the point of junction of mid- and hindgut the four Malpighian tubules enter the alimentary canal. The hindgut consists of three parts, of which the first and third are narrow with a low epithelium. The second part of the hindgut is short, wide, and almost completely surrounded by six rectal papillae provided with a rich tracheal supply. Each of these is a syncytium, a tissue formed by the fusion of many cells the divisions between which cannot be distinguished; the surface of the syncytium covers a large area of the lumen, and is in direct contact with the contents of the hindgut (Sikora, 1916). Anatomy and structure suggest that these organs are rectal papillae; in insects of many groups their function is the removal and recirculation of water from the contents of the hindgut. Observations on the living louse confirm this. When the insect begins to feed, a small quantity of blood may be passed quickly from mid- to hindgut, indeed, drops of undigested blood may appear at the anus. But later in digestion the material is kept longer in the hindgut. In the anterior part it is fluid, but in the second part of the hindgut which is surrounded by the rectal papillae, water is extracted from the faeces, which are passed as dry pellets which frequently adhere to one another in a string. When digestion is completed, the papillae also extract water from the clear excretion of the Malpighian tubules and concentrate it till it is semi-solid (Wigglesworth, 1932).

3. Salivary Apparatus

The salivary apparatus is complex. There are several minute glands in the lower part of the head, the ducts of which open into the sheath of the stylets; though they are generally described as "salivary", their function is not known. There are also two pairs of glands in the dorsal part of the thorax; of these the anterior are reniform, the posterior tubular ("horseshoe shaped"). Both lie close to the anterior part of the midgut (figure 15). The ducts pass forward into the head and unite inside the stylet sheath to form a median duct, continuous with the intermediate stylet (figures 12 and 14); it is clear from this that these glands are salivary. It has been shown that if the reniform glands are dissected out, emulsified and injected into the human skin, they produce irritation 8-IO hours later, followed by a bluish swelling; the tubular glands produce no- such effect (Pavlovskyi and Stein, 1925). It seems, therefore, that the irritation caused by the insect's bite is due to the secretion of the reniform glands alone.

In front of the reniform salivary glands there is a group of large cells, generally binucleate, containing greenish droplets. There are groups of similar cells in several parts of the fat body. These cells have been described as "nephrocytes." It is now held that the function of such cells, which occur in many types of insect is analogous to that of the reticulo-endothelial cells of vertebrates; they are concerned with the "micro-phagocytosis" of colloidal particles (Wigglesworth, 1939, page 238).

4. Mycetome

Reference has already been made to the mycetome or stomach- disc which lies mid-ventrally in the wall of the anterior part of the midgut (figure 15). The organ is rounded, yellowish and readily seen through the ventral integument of the living insect. Towards the end of embryonic life it arises as a pouch in the wall of the midgut, from which it eventually becomes completely separated; its cavity is crossed by radial septa. The cavities between the septa are full of short rods (figure 16, A), which are probably micro-organisms, and which are generally described as "symbionts." A study of the unhatched embryo shows these symbionts lying in a group of cells in the yolk which then fills the lumen of the gut. These cells approach the mycetome at the period when it is a pouch opening into the lumen of the gut, and the symbionts enter it (figure 16, B). There they remain throughout the life of larva and male louse. But in the female the majority of them leave the mycetome about the time of the last moult and migrate to the wall of the oviduct; from that position they enter the egg, and so reach the embryo, thus completing their cycle.

In shape the symbionts resemble bacteria; as they retain their characteristic individuality and appearance during this remarkable life-cycle, it is impossible to suppose that they are a part of the louse's tissues, and the view that they are micro-organisms is extremely probable.

Admitting that these rods are micro-organisms, are they parasites or symbionts? The fact of their invariable presence in the louse and of the hereditary transmission suggests that they are symbionts. This is supported by experimental work, for it is possible to remove the whole mycetome by open operation. If this is done on an adult female after the symbionts have migrated to the oviduct, her length of life and powers of laying fertile eggs are little affected; it may therefore be concluded that the mycetome without its contents is not of great value to the insect. But if the mycetome is removed from the female very early in her adult life, while it still contains symbionts, she lives only a few days and lays eggs which are malformed and not viable: removal of organ and symbionts from larvae also causes death. The same result may be obtained by centrifuging the egg and displacing the mycetome. If this is done before the symbionts have entered it, the larva only lives a few days. Clearly, then, the micro-organisms are necessary to the insect's survival and reproduction; i.e. they are beneficial to the insect, and are therefore rightly described as symbionts (Aschner, 1934). It is not known precisely how they are of value to the louse, but work on symbionts of other insects which feed entirely on blood throughout the life cycle suggests that symbionts are micro-organisms which supply vitamins deficient in the blood which forms the louse's sole diet. (For a general account see Wigglesworth, 1939: for recent work with Rhodnius, a blood- sucking bug, see Brecher and Wigglesworth, 1944.)

FIG. 16. Pediculus humanus, mycetome in ventral wall of midgut. (X750.)
A, in larva (male), showing mycetome lying between epithelium of midgut (mg) and ventral wall of abdomen (st). The symbionts lie in groups separated by septa.
B, in embryo, showing primary mycetome now degenerating (pm) among yolk cells in gut (y), and symbionts in definitive mycetome (my) in gut wall. (After Ries.)