386 LtlCTURE XVIT. 



abdominal segment, and projecting toM'ards the back; in the Semblidce 

 plumose branchiae are similarly situated. In SialidcB the branchiae are 

 four or five-jointed, and are said to aid in swimming. In the larval 

 Mayflies {EphemeridcB) each of the anterior abdominal rings has a pair 

 of branchiae, sometimes ramified, sometimes of two kinds, one fasci- 

 culate, the other laraelliform, and performing oscillatory movements. 

 Certain larval dragon-flies {Agrion, Calopteryx) have three long 

 lamelliform branchiae standing vertically on the hinder part of the 

 abdomen : others {^sthna, Libellula) have numerous epithelial folds, 

 including many fine branches of tracheae, projecting into the Inrge 

 rectum, the outlet of which is provided with valves to regulate 

 the entry and escape of the water required for respiration. Rosel* 

 has beautifully figured the branchiae of the Gyrinus, or whirligig 

 water-beetle. In a large N. American neuropterous insect (Ptero- 

 narcys regulis) the branchiae, developed from the lower or sternal 

 arcs, are persistent, and co-exist with the modified branchiae from 

 the upper arcs, which form the wings. They consist of eight pairs 

 of sacs, supporting numerous long setose filaments, which form a 

 thick tuft from the exterior of each sac ; and are situated over the 

 stigmata at the infero-lateral parts of the thorax and anterior seg- 

 ments of abdomen ; each filament is traversed by a tracheal vessel. 

 The branchi-tracheal system is more simple in the larvie of Tipu- 

 lidce, in which the tracheae that transfer the air from the water are 

 subcutaneous. In all the modifications, the tunics of the branchial 

 tracheae effect that transference by actions analogous to endosmose 

 and exosmose.f 



The amount of respiration is directly as the degree of the activity 

 of the insect ; and its temperature is increased in an approximate 

 ratio. The extraordinary development of the breathing organs de- 

 monstrate their essential relations to the energies of the muscular 

 system; and, by a minor modification, they are made subservient to 

 the diminution of the weight of the insect. In the Apterous insects, 

 and especially the Myriapods, there is no trace of air vesicles, but 

 both in the Centipede and lulus the minute tracheae ramify throughout 

 the body. 



The powerful and often disagreeable odour emitted by certain 

 insects in their larval or winged state, is secreted in most by subcuta- 

 neous follicles, and excreted at particular pnrfs of the body. The 

 larvae of the Coccinellidce and 7^e?itliredinidcc exude such fluid 

 secretion from pores diff'uscd over the surface of the skin. In the 

 water-beetle {Dytiscus) a nauseous fluid escapes from the crphalo- 

 thoracic joint ; in many Mdo'idce and ChrysomcHdip, it escapes from 



CV. Tb. ii. tf. 23. t XXIV. p. CAX 



