INSECTA. 



385 



in Nepa, by both, into a large longitudinal tracheal trunk {g), which 

 runs near each side from one end of the body to the other ; they are 

 connected together by transverse 

 tubes, which run across the pos- 

 terior margin of each abdominal 

 sejrment, and distribute an in- 

 finitude of smaller tracheal ra- 

 mifications. Some of these 

 branches dilate into air recep- 

 tacles (//), the number and size 

 of which, like the air-cells in t^ 

 birds, are in direct relation with 

 the powers of flight. In the 

 Nepa these reservoirs of air 

 are confined to the thorax : in 

 other insects, as the grasshop- 

 per, they are frequently deve- 

 loped also upon the transverse 

 abdominal tracheae : they are 

 very capacious in the abdomen 

 of the bee. 



The spiracles are narrow 

 two-lipped orifices, situated at 

 various points on the external 

 surface of the body, and differ- 

 ing in number and size in diffe- 

 rent insects. In the Coleoptera 

 there is a spiracle at the inter- 

 space between every two segments : the Diptera have the fewest 

 spiracles ; they are reduced to two, e. g., at the extremity of the ab- 

 domen in the CEstrus larvaj. In some insects the orifice is situated 

 upon an entire oval horny ring. In many insects, especially those that 

 burrow, the margins of the spiracles are defended by a fringe of hairs, 

 wliicli prevent the entry of extraneous particles. In the larvce of the 

 lamellicorn beetles, the orifice is closed by a sieve-like membrane, 

 admitting the air by the marginal pores. In the mole-cricket, the 

 thickened margin of the spii'acle is strengthened by two horny 

 half rings, and can be closed by the action of a small sphincter 

 muscle. 



Most of the aquatic larvae breathe by temporary gills {branchice 

 tracheales) ; these consist of membranous cylinders or lamina? contain- 

 ing tracheal tubes. In the Phryganeaj they are filiform, united in groups 

 of from two to five, developed from each side of the upper part of each 



c c 



Respiratory system, Nepa. 



