If the abdominal terga are removed as a connected piece, the heart and surrounding tissues may be found on its inner surface. The heart consists of a tube, divisible into an anterior aortic membranous portion, which runs forward into the thorax, and a posterior muscular portion ending blindly behind, and divided into a series of chambers by lateral apertures. Its muscles are arranged circularly or spirally, and at the apertures in a figure of 00 . The apertures are valved. The chambers contract successively from behind forward, and according to Cornelius, there were eighty such contractions in a minute in a Blatta which had just undergone ecdysis. The heart is suspended to the back of the abdomen by muscular fibres. Its walls are connected to a network of elastic fibres and interposed pericardial cells, which resemble the cells of the fat body, and the whole is limited on the surface turned to the viscera by a tissue partly fibrous and partly composed of the paired alary muscles. These muscles are striated, and arranged segmentally in fan-shaped bundles. The handle of each fan is attached laterally to one of the terga; its expanded portion is spread out below the heart, and the muscle-fibres end in tendons reticularly arranged.
The whole structure, as pointed out by Graber, forms a pericardial sinus, which expands and contracts rhythmically like the heart. Numerous tracheae ramify in it and upon the heart. A very similar structure, with alate muscles, covers over the ventral nerve cord, and forms a pulsating sinus, but the contractions run from the anterior to the posterior extremity.
There are nine stigmata, or respiratory apertures, two thoracic, and seven abdominal. According to Bela Deszo, the stigmata and apertures into the heart correspond numerically in Insecta, Myriapoda, and Arachnida. This statement can, however, only be true as far as concerns the abdominal stigmata. The two thoracic stigmata are situated, one in the meso-, the other in the meta-thorax, in front of the articulation of the coxa. The abdominal stigmata are placed immediately under the lateral expansions of the terga, upon conical papillae. With the exception of the first, these papillae are concealed by lateral prolongations of the sterna. The last of the series is the largest. The entrance into the trachea, which rises from each stigma, is protected (1) by hairs which cross the aperture, (2) by a special apparatus for closing the tube, which in the Orthoptera is continuous with the lips of the stigmatic aperture. These lips are prolonged inwards as two valves. A process arises on the outer, i.e. visceral surface of each of these valves. A muscle passes from one to the other process round one of the margins, and when it contracts, squeezes the valves together, and thus narrows the aperture. The details of the structure appear to differ somewhat in the thoracic and abdominal stigmata.
In most Insecta the apparatus for closing the trachea is separate from the stigmatic lips. It is apparently absent in the Rhynchota and abdominal stigmata of Diptera.
The tracheal wall consists (1) of a layer of polygonal cells continuous with the hypodermis of the body walls; (2) of an external supporting membrane; and (3) of an internal chitinous coat or intima secreted by the cells. This chitinous coat, except at the ultimate terminations of the tracheae, and in certain tracheal dilatations, is marked by fine transverse lines, which, as usually explained, are due to a spiral thickening of the coat with intervening thin membranous portions. But Macloskie has adduced reasons for believing that the spiral thickenings are really spiral crenulations, i. e. tubular or channel-like folds open to the trachea by a slit or fissure. He points out that such an explanation is in harmony with the structure of the pseudo-tracheae in the proboscis of Musca, and would also account for the lengthening and shortening of the tracheae themselves during the respiratory expansions and contractions of the abdomen.
The testes undergo atrophy in the adult male. In the wingless, i.e. immature, male they are to be found as numerous pyriform vesicles placed dorsally, as is usually the case with the genitalia of Insecta. They are attached by short pedicles to a common duct. The ductus ejaculatorius opens on the 10th sternum. It is dilated anteriorly, and to the dilated portion two glands are attached, - one, a mushroom-shaped gland composed of short caeca with viscid granular white contents, the other composed of dichotomous moniliform tubes, united by a common investment into a long mass overlying the last ganglion. The spermatozoa have straight rod-like heads and long flagella or tails.
Hauser has recently investigated the function and structure of antennae. He concludes upon experimental grounds (effect of strong-smelling substances; the power of finding odorous food or in certain instances the female; according as the antennae are present or removed) that antennae possess an olfactory function. The sensory apparatus consists (1) of a conducting antennary nerve arising from the supra-oesophageal ganglia in connection with certain lobes (Bellonci, Flogel); (2) of a hypodermis-cell, in union basally with a nerve-filament, and terminated by a freely projecting rod, but varying in other respects in different groups; (3) of a supporting and protective apparatus in the shape of either a groove or a chitinous cone, open round the base of the projecting sensory rod to the cavity of the antennae, and hence filled with blood plasma. The cone is generally open at its tip. The groove is in some instances (Orthoptera, Apis) closed by a superficial delicate membrane but when it is freely open, escape of liquid is impossible, by reason of the minute size of the aperture. In the Diptera a number of sensory cells are contained within one common depression. In some Hymenoptera, grooves and cones occur side by side.