The visual (retinal) cells possess visual rods at their inner ends, and the surrounding cells are pigmented at their peripheral ends, hyaline at their inner ends, which bend inwards horizontally over the visual cells. The structure requires examination in other larvae. In the ocellus of the imago the ommateum is diplostichous, the vitreous cells are generally small and flat, and the rods of the visual cells composed of two refractile plates covering the anterior (or inner) fourth of the cell. The shape and size of the lenses in the polymeniscous eye vary even sometimes in one and the same eye, e. g. in Odonata. The vitreous cells either persist in a uniform layer (the acone eyes of Coleoptera minus Pentamera, Rhynchota, Tipularidae among Diptera, Dermaptera), or are grouped into fours forming vitrellae, and then (I) surround a clear fluid or gelatinous cone (the pseudo-cone eyes of Diptera Brachytira, e. g. Musca), or (2) are reduced to feeble remnants inclosing a solid crystalline cone composed of 2-5 parts, the eucone eyes of other Insecta, including Lepisma. The visual cells are grouped to the number of seven into retinulae. In acone eyes one cell stands in the centre with the six others around, and the visual rods or rhabdomeres are contained one in each cell.
In pseudocone and eucone eyes the seven cells are grouped round a central axis. The rhabdomeres in the former coalesce only anteriorly, in the latter throughout their whole length, into a rhabdome. Pigment cells lie between the vitrellae and retinulae separating the ommatidia from each other. So far as observations go the compound eye is autochromic. There are as a rule two optic ganglia, a peripheral and a central. Details vary much1. Auditory organs a longer or shorter peduncle (imago of Lepidoptera, Diptera, many Neurop-tera). The stomodaeal section is usually straight and traverses the thorax. The mesenteron and proctodaeum lie in the abdomen in the imago and are more or less convoluted, the coils being held in position by tracheae, fatty tissue, and nerves. The mesenteron or chylific stomach varies in length. Its anterior extremity may develope a few caecal tubes (many Orthoptera, Plecoptera). Its epithelium contains glandular cells, and its outer surface is sometimes beset with villiform glands (some Coleopterd). The proctodaeum does not communicate with the mesenteron in the larvae of aculeate Hymenoptera, some Neuroptera (Myrmeleo, Hemorobiidae), and Pupipara among Diptera. In the Neuroptera named it forms a silk gland.
It is generally of some length and its calibre varies in different regions. The short posterior dilated section or rectum is generally, especially in the imago, provided with 2-6 longitudinal ridges richly supplied with tracheae. These ridges become of functional respiratory importance in the larval Odonata, in which water is taken into and expelled from the rectum rhythmically. The anus is terminal in the last abdominal somite.
1 Patten draws attention to the following points (Mitth. Zool. Stat. Naples, vi. 1886): (1) As to the ocellus of the imago. The vitreous cells are, properly speaking, corneal hypodermis cells; the retinophorae are terminal; and the layer of visual cells is a retineum in which the retinidia of the retinophorae form a continuous layer. (2) As to the compound eye. It has the typical structure detailed in note, p. 452. In Mantis religiosa, the type which he examined, the retinidia = crystalline appear to occur in two forms: (1) as vesicles containing some clear floating globules, found only in some Diptera in the posterior region of the abdomen in the larvae (Tabanus, &c), or in the basal joint of the antennae of the imago; (2) as the chordotonal organs. The essential structure of these organs consists of a nerve ending in a ganglion, the cells of which are provided with terminal processes or end-organs, composed each of a terminal rod (' Endstift') variously conformed and inclosed within a sheath or 'scolopophore.' The end-organs are either connected directly to the hypodermis or indirectly by a lateral filament. They appear to occur in the larva as well as the imago, and in all orders of Insecta except Thysanoptera, where they have not been detected.
They are found in groups of 2-200 in various parts of the body, antennae, palpi, legs, wings, in the halteres of Diptera, and upon the dorsal aspect of the abdomen. Among the Orthoptera, they are connected in Acrididae with a thin membrane, of which there are two, one on each side of the first abdominal somite; and in Locustidae and Gryllidae they lie in a special pit under a thin membrane on each side of the proximal ends of the tibiae of the fore limbs. The main trachea of the limb is dilated between the two organs. Cells furnished with a pointed projecting rod, and connected to a nerve, are found in various Insecta on the antennae, and on the palpi, and in various parts of the oral cavity, e. g. in the Bee. In the first two instances they are supposed to be olfactory, in the third gustatory in function.
Salivary glands, varying in shape, number, and structure, open into the oral cavity, the most important pair belonging to the labium. They appear to be developed close to the mandibles and not from the stomo-daeum. Silk glands or serictaria open on the labium in the larvae of Lepidoptera and the Tenthredinidae, etc. among Hymenoptera. They are developed close to the inner side of the labial limbs. The oral cavity is closed in the larvae of Dytiscus and of Megaloptera among Neuroptera (e. g. Myrmeleo, Chrysopa) which suck up the juices of their prey through the perforated mandibles, in the imago of Epkemeridae and male Coccidae and Aphidae. In the last-named family, the male and perfect female Phylloxera are stated to want the digestive tract. The stomodaeum varies in complexity. It may dilate into a crop and the muscular walls of its posterior part become much thickened and the chitinous lining ridged and toothed, forming a gizzard (Orthoptera, some Coleoptera, &c). It may be provided with a dilatation or so-called sucking stomach, attached by cones are fluid, and the style (= rhabdome) of the four retinophoral cells does not expand into a basal pedicel.