14. On the Internal Anatomy of the Female Lac Insect, Laccifer lacca Kerr (Homoptera: Coccidæ) *

Summary.1 The integument of the lac insect consists of an external layer of cuticle, underlying which there is the syncytial hypodermis, many of whose cells are modified into lac glands which secrete the resin of commerce. 2 The lac glands are of two types. The first type includes pleuricellular glands which are restricted in their distribution to the perivaginal pore clusters. To the second type belong unicellular glands with separate ducts and prominent nuclei. Some of these are very much limited in their distribution, being present only along six serpentine areas of marginal duct clusters. These are flask‐shaped in appearance, and have wide necks and prominent nuclei and nucleoli. Others belonging to the same category are distributed diffusedly all over under the integument, excepting the six serpentine areas, and possess a round fundus with filiform ducts. 3 The gut of the lac insect consists of a club‐shaped pharynx, an elongated œsophagus, a much‐convoluted ventriculus, a recurrent intestine, a colon with a cæcum, and, lastly, the rectum. A pair of salivary glands are also present. 4 The pharyngeal wall is made up of a single layer of columnar epithelial cells with prominent nuclei. The greater part of it lies within the chitinous framework of the mouth, and at its oral end are present a pair of salivary glands whose ducts open into it. The œsophagus is a long, narrow tube of the same histological composition as the pharynx, and is drawn in for some distance into the interior of the colon, where it forms the convoluted ventriculus. The wall of the ventriculus consists of a layer of large conical cells the free ends of which bulge out towards the lumen. At the base of these cells is a prominent deeply‐staining nucleus. This association of the ventriculus and the colon is a very interesting feature. The convolutions of the ventriculus are independent of the colon. The recurrent intestine may be said to consist of a proximal and a distal limb, the junction of the two being marked by the attachment of the Malpighian tubes. The wall of the intestine is thrown into folds, and the lumen is lined with a layer of chitinous intima. The wall of the colon consists of a layer of moderately elevated conical cells. The cellular nature of the wall of the colon is suppressed in the zone enclosing the convolutions of the crop. The rectum has a very thin wall. A few prominences in its wall are the only indication of its cellular character. The lumen of the rectum is lined by a chitinous intima which shows a few ridges. The Malpighian tubes, of which there is a pair, present a beaded appearance due to the distention of their cells with excretory matter. They are made up of two alternating rows of cells. 5 In common with most other Coccids, the female lac insect also produces large quantities of “honeydew,” which is sought for by the ants. The belief that honeydew is secreted for the ants, and that the ants, in return, are of service to the lac insect in warding off their enemies, does not seem to rest on any positive evidence. This question has been discussed thoroughly in the preceding pages, and it has been shown that the honeydew is dependent upon a vital process for its production, and is not produced for the ants or, for the matter of that, for my other insect. Arguments have been brought forward negativing the belief in the symbiotic relationship between ants and the female lac insect. The prevalent belief that ants are injurious to a successful cultivation of lac has been shown to be without foundation. 6 The author has shown that the dorsal spine is not capable of discharging any of the functions hitherto assigned to it by various authors. At the same time it is difficult to imagine what function it could possibly discharge in the female lac insect. 7 The female lac insect possesses two pairs of spiracular openings. The anterior pair of spiracular openings is borne by the brachia, which are grooved on their outer margins to lodge the spiracular openings of the respective sides. Each spiracular opening leads into the caværa, from the base of which the main tracheal trunk arises, which, by further subdivisions, gives rise to a bushy tuft of trachea. The caværa is bounded by the peritreme, which bears numerous pores. The posterior spiracles are situated near the oral end of the insect, and, though built on the same plan, are smaller in size and lack muscular slips. There are trachea in association with these also. 8 Through the dimples of the brachial plates and the anacerores of the anal tubercle waxy threads are given off. It has been shown that these are not the attenuated extremities of the tracheæ, being formed by wax glands situated under the areas referred to, and hence of a waxy character. The structure of the wax glands has been dealt with, and they have been shown to resemble the wax glands of Pseudococcus and Ceroplastes in many respects. 9 There are three bundles of muscles. Two of these are attached near the mouth‐parts on the one hand, and, on the other, are inserted in the interspaces between the anal tubercle and the brachia of the corresponding sides. The third bundle is inserted at the oral end near the æsophagus, on the one hand, and, on the other, extends into the anal tubercle, which it muscularizes. The correct morphological value of these muscular bundles has been indicated in the text. 10 No vascular vessel or pulsatile vessel was detected in the female lac insect. 11 The nervous system is represented by two separate ganglia, one of which is situated close to the colon‐cum‐rectum portion of the gut; the other lies close to the Malpighian tubes. The first of these represents the thoracico abdominal ganglion of the larva, whilst the other is the cerebral ganglion. The thoracico‐abdominal ganglion gives off a few nerves which innervate the interior of the insect. 12 The ovary of the female lac insect is an arboreal organ bearing numerous diverticula in which the embryos subsequently undergo development. The oviducts of opposite sides unite to form the uterus, which leads into the vagina. The female genital opening is situated on the anal tubercle. The receptaculum seminis consists of a stalk and a terminal chamber, and is attached to the oviducts just a little above the point where the two meet. 13 The wall of the ovary consists of a thin cellular layer. The oviducts have the same essential structure as the ovarian wall. The wall of the uterus consists of an internal layer of columnar epithelium resting externally on a layer of circular muscles external to which are the longitudinal muscle fibres. The vagina has the same structure as the uterus, but differs from it in having the cells of the epithelial layer more elongated and closely packed up on account of its being thrown into folds and in having the lumen lined by a chitinous intima. The receptaculum seminis has the same histological structure as the uterus, but no cell boundaries are distinguishable in its internal epithelial layer. 14 The oviducts open to the exterior quite independently of the rectum on one side of the anal tubercle. 15 The larvæ emerge through the female genital opening and not through the anus, spiracle, or by the rupture of the skin as described by many previous workers. 16 The death of the female lac insect seems to be due to the incapacity of the Malpighian tubes to store up any further quantity of waste matter when its cells, in the advanced stage of life, have become distended to the utmost. The Malpighian tubes, from larva onwards, gradually become laden with waste matter until they become distended to the utmost, when death, due to autointoxication, results. Similar instances in the Animal Kingdom are provided by the Ascidians and the Anurida.