INSECTA. 505 



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. 



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 (Pprothorax) 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. Heteroptera) ; 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 portions 1 . The crenulations disappear 

 in the ultimate tracheal capillaries, which either end in certain cells con- 

 tained 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, 



1 Packard states (Amer. Naturalist, xx. 1886, p. 440) that the spiral appearance is decep- 

 tive. 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. 



