This last-named structure is perhaps more or less characteristic of nocturnal Insecta. The red light which is reflected outwards from it, is especially well seen in Moths. It is evident that Grenadier's acone eyes in particular will require re-examination in the light of Patten's researches.
Excretory organs or Malpighian tubes open into the commencement of the proctodaeum, rarely into the end of the mesenteron as in Thysanura, Termes, etc. They arise from the proctodaeum as one or two pairs of outgrowths, but their number may be subsequently increased by budding. The full number present may be small (2-8) or large (30-50 or more, e. g. 150 in Apis). Hence Insecta may be designated as Oligo- or Poly-nephria. They may branch, and they open into the proctodaeum separately or united in bundles with a common aperture.
The coelome is more or less filled by the viscera and fat-body when present. The latter is always plentiful in the larva, and persists to a variable degree in the imago. It is a tissue composed of fat-containing cells richly supplied with tracheae. The blood is colourless or yellowish, tinged with green in vegetable-feeders, or sometimes reddish. In the larva of Chironomus (Diptera) it is deeply coloured with Haemoglobin. The blood corpuscles are amoeboid 1 The heart is placed dorsally in the abdomen. It consists in the imago usually of eight chambers with lateral valved ostia. The anterior end gives off an aortic trunk which extends to the head. The posterior end is closed, and but rarely gives off vessels. The organ is suspended by muscular filaments to the terga, and lies in a pericardial sinus limited ventrally by the alary muscles, the contraction of which dilates the sinus and affects the heart indirectly. The sinus is partially filled by pericardial cells and connective tissue, and the blood enters it between the alary muscles.
There is a ventral sinus of similar structure and similarly pulsatile; open in front and behind, and lodging the nerve-chain.
1 Von Wielowicjski distinguishes three types of blood-cells in Insecta. (1) Fatty cells, constituting the fat body; they are grouped together, are rarely bi- or multi-nuclear, and their protoplasm is filled with fat drops. (2) Oenocyths, multi-nucleate cells with slightly coloured protoplasm and containing but few granules, tied to the smallest tracheal capillaries. (3) Pericardial dells, which differ much in character but are known by their position in the pericardial system. Z. W. Z. xliii. 1886.
Respiration is tracheal. The majority of Insecta are holopneustic, i. e. possess open stigmata. Each stigma leads into a single tracheal stem, rarely into several. A pair of stigmata lie in the head (?prothorax) of Smynthurus (Collembola), and there are in the embyro Lepidopteron three pairs in the same region. Otherwise stigmata are restricted to the thorax and abdomen. The latter possesses eight pair as a maximum, but the number may be reduced. There may be a pair on the prothorax, e. g. Lepidoptera, Coleopterous larvae; a pair on the meso- and meta-thorax, e. g. Hymenoptera; or on the pro- and meta-thorax, rarely on all three (Siphonapterd). The stigmata lie in the thorax above the base of the limbs, in the abdomen, either in the soft pleural membrane or between successive somites, and are either freely exposed or concealed, e. g. by the elytra in Coleoptera, or the overlap of the somites in Hymenoptera. In various aquatic larvae and some imagines of Heteroptera, e.g. Nepa, there is a posterior pair of stigmata to which air is brought by a simple or split tube. The stigmatic aperture consists either of a simple slit with a surrounding chitinous ring (e. g.
Heteropterd); or of a number of apertures leading into a common tracheal stem (Dipterous larvae and pupae); or it is provided with more or less prominent lips, and may then be protected by hairs, e. g. Coleoptera, Lepidoptera. The commencement of the trachea is closed by a special apparatus of chitinous structures and muscles controlled by the nervous system. The tracheae are frequently connected close to their origin from the stigma by lateral longitudinal trunks; they branch and anastomose, and the finest branches are distributed to the muscles, nerves and other viscera. In insects of great powers of flight the branches have vesicular dilatations. The system of tubes is lined throughout by a chitinoid membrane, plain in the dilatations, crenulated spirally in the tubular portions1. The crenulations disappear in the ultimate tracheal capillaries, which either end in certain cells contained in the coelome or in the investing membranes of the muscles and other organs. When the tracheal system becomes completely closed, the Insect is said to be apneustic (larvae of Ephemeridae, most Odonata, of the Plecoptera, Trichoptera, and of Corethra among Diptera). In this case the rectum is respiratory (Odonata); see supra.
Or leaf-like processes are appended to the end of the abdomen (Agrtonida, among Odonata); or similar leaf-like, filamentous, or tufted processes are attached laterally to more or fewer of the abdominal somites (Ephemeridae, Calopterygidae among Odonata, Trichoptera, Sialidae among Neuroptera); or to the thorax (Plecoptera), and then they are often retained by the imago side by side with the stigmata 1 These tracheal gills rise close to the rudiments of the stigmata, which appear to be always present. There is a longitudinal tracheal trunk from which tracheae pass off to the gills on one side, the viscera on the other. The condition of an apneustic insect has been secondarily acquired.
1 Packard states (Amer. Naturalist, xx. 1886, p. 440) that the spiral appearance is deceptive. The apparent spiral lines or 'taenidia' of the tracheal membrane ('endotrachea') are due to the fact that the outer cell layer ('ectotrachea') proliferates during the formation of the tracheal tube, and forms an inner layer of endotracheal cells (or nuclei ?). These inner cells lengthen out into parallel band-like processes which unite laterally, constituting the lining intima. The median portion of each band persists as a taenidium.