The Internal Anatomy of the Imago.
The Alimentary Canal.
The alimentary canal morphologically resembles that of the nymph in its division into various parts, but differs in several respects.
The Oesophagus occupies the same position as the nymphal. Posteriorly, however, at its junction with the crop, it constricts and its epithelium and intima are thrown into a large number of folds.
so that the lumen is almost obliterated. This region is surrounded by a large amount of fat.
The Crop is normal in position, but differs in that it is greatly distended with a gas the composition of which is unknown.
The Gizzard in the imago has increased in length and extends backwards as far as the fourth abdominal segment. Like the crop, it is distended with gas, this being prevented from escape into the mesenteron by the oesophageal valve.
The Mesenteron is shortened correspondingly with the elongation of the gizzard, and now lies in the fifth and sixth abdominal segments. It is much reduced in diameter and is pigmented posteriorly. The parasites referred to in the section on bionomics are extremely common.
The Hindgut is very much reduced in diameter, but displays the same structure as in the nymph. Posteriorly there is a constriction dividing it into ileum and colon-rectum, in the latter of which a trace of faece was detected.
Secretory and Excretory Glands of the Gut.
The Salivary Gland.—The salivary glands resemble those of the nymph in every respect.
The Malpighian Tubules.—These structures resemble those of the nymph except that they are of a darker yellowish brown suggesting a greater deposition of solid urates. Similarly the tubules pass forwards and are attached to the gizzard. They are all well supplied with tracheae, lie in close relation to the gonads, and arise from the mesenteron in two bands, each tubule opening into the latter by a separate opening situated on a small papilla.
The Nervous System.
The nervous system is identical with the nymphal.
The Circulatory System.
This is identical with what has already been described.
The Respiratory System.
The general nymphal respiratory plan is adhered to (Pl. 37, Fig. 24), although slight modifications have taken place to adapt it to a terrestrial mode of life. It has been shown that in the last larval instar the thoracic spiracles were present but closed; now, however, they are functional, and eight functional abdominal spiracles have been added. The main longitudinal trunk and its branches are still present, as are also the typical Y-shaped tracheae to the leg. In each of the first five abdominal segments, branches still pass downwards and outwards to the vestigial gills, a discussion of which has already been given. Each branch again sends a trachea to the gut, the muscles, and fat body, but with this difference that the large branch to the former is now connected with its fellow of the opposite side, to form a ventral commissure not found in the nymph. This condition occurs in the second to fifth abdominal segment, whilst the
first segment has a slightly different arrangement described below. Intersegmentally the longitudinal trunk gives off eight short, spiracular, unbranched tracheae to the first eight abdominal spiracles, except that the spiracular trachea of the first arises by the bifurcation of the tracheal trunk to the first gill, one branch of which passes to this latter, the second to the spiracle, whilst the third forms the posterior stem to the leg. This latter shortly after its origin from the spiracular trunk gives off a branch internally to join a similar branch from the opposite side, forming the first ventral commissure. This commissure is connected to the second on either side by a small longitudinal trachea running parallel and ventral to the main trunk. This again is not found in the nymph, and may or may not be a variation. In the specimen dissected the trunks to the wings were severed, since dissection was performed from the dorsal surface, but the cubito-anal branch to the forewing could be traced, and was found to be normal in position. From this there seems no reason to doubt that the basal tracheae to the legs and wings are the same as for the last nymphal instar. The atria of the thoracic spiracles are now much more marked than in this latter, and it is from these that the basal tracheae to the wings arise, the trachea having migrated along the leg stems to these regions.
In the mesothorax the main longitudinal trunk gives off a fairly large branch to the muscles, represented only by a single small branch in the nymph. Together with this are the smaller branches to the muscles, the increased tracheation being correlated with the increased activity of the wing-bearing segments.
The atrium of the first spiracle is deviated forwards and constitutes that portion of the main trunk lying in the pro-thorax. This by bifurcation gives rise to the dorsal and ventral head trunks, the latter of which gives off a large branch to the legs as it passes forwards. The former resembles that of the nymph, except that at the point where it curves outwards in the head, a large trachea is given off. This curves forwards and inwards to meet its fellow from the opposite side, forming a dorsal, anterior commissure between the two longitudinal trunks, a condition which is absent in the nymph.
The development of commissures is no doubt correlated with flight, this rendering it necessary to have direct communication between the trunks, thus making it possible for all parts to be oxygenated more rapidly.
The Reproductive System.
The Male Reproductive Organs.—These consist of a pair of long, convoluted tubes, the testes, extending forwards to the metathorax, where each joins its fellow from the opposite side (Pl. 40, Fig. 35). In a freshly dissected specimen each tube is seen to have an inner, opaque, central portion, and an outer transparent. Each tubule lies more or less dorso-laterally to the gut, and extends backwards. passing imperceptibly into the vas deferens. In the eighth segment
each vas deferens descends and runs backwards to the posterior region of the ninth, where it doubles back on itself, running forwards into a large vesicula seminalis (Pl. 40, Fig. 36). Each of these is joined to its fellow anteriorly, is pear-shaped, convex above and below, and opens into the ejaculatory duct by an opening of its own. There are a pair of accessory glands lying immediately beneath the vesicles, each of the former opening into the corresponding vesicle of its side. In shape they are irregular cylinders communicating with one another anteriorly. The accessory gland is revealed to be of the mesodenium type of Escherich.
The ejaculatory sac lies immediately above the hypandrium, and appears to have a portion of the median unpaired aedeagus together with the ductus ejaculatorius retracted within it.
The Female Reproductive System.—The ovaries are of the panoistic type in which no nutritive cells are present; they are two in number and extend backwards from the first abdominal segment to the eighth, lying dorso-laterally to the gut, which is much reduced in size. Each ovary has a long, straight tube, the oviduct, from which on either side are given off a large number of ovarioles (Pl. 40. Fig. 37), which increase in length from before backwards until a maximum is reached. Anteriorly on the mid-dorsal line each ovary joins its fellow. Each ovariole may be divided into three regions:—
(1) the terminal filament;
(2) the germarium;
(3) the vitellarium.
The Terminal Filament is a minute, slender prolongation of the peritoneum which invests each ovariole. All the filaments of an ovary are bound together, and anteriorly where the ovaries join they all meet and continue forwards as the median or dorsal ligament, which finds attachment on the pericardial diaphragm.
The Germarium (Pl. 40, Fig. 38) in the youngest ovarioles-appears as an undifferentiated portion containing large nuclei with nucleoli. From this region the ova and follicular epithelium are developed. In older ovarioles the germarium shows the ova differentiated in linear series; this condition appears to be present in the ovarioles nearer the gonopore, the more anterior ones seemingly being of a younger nature.
The Vitellarium (Pl. 40, Fig. 37) forms the major portion of the ovariole and has the ova disposed in a linear series, each ovum being enclosed in a definite follicle and surrounded by a layer of follicular cells, which, as the ova pass down the ovariole, secrete the chorion. Meanwhile, before this occurs, the ova increase in size by the accumulation of oil droplets. Each ovum has a large nucleus.
Each oviduct extends backwards so that the ovarioles lie on top, and on the sides of the gut, and in the intersegmental region between the seventh and eighth segments, each duct descends to
the midventral line, where both join up and pass forwards for a short distance to open on the pit-like depression already described. There is no uterus nor is there any vagina, the cuticle not being invaginated into the common oviducal duct, which according to some authorities is not a primitive condition.
An ovoid accessory gland lies in a posterior position to the common oviduct. It opens to the exterior on the pit-like depression into which the oviduct opens. Presumably it is an albumen gland, for the eggs when freshly laid are surrounded by an oval-shaped mass of transparent substance, which disintegrates after about twenty-four hours.