THE COMMON COCKROACH 141 



character and extent by the ingenious method of pro- 

 jecting the form of the body of the living insect on a 

 screen by means of the lantern, and then tracing its 

 outline during inspiration and expiration respectively. 

 In general, an insect at rest performs its respiratory 

 movements with the hinder part of its body, in other 

 words, it pants with its abdomen, the movements con- 

 sisting of an alternate contraction and recovery of shape 

 of that region. Amongst British insects there is perhaps 

 no species in which it is easier to watch these movements 

 than the great green grasshopper, a large locust-like 

 insect, found not un frequently in some parts of the 

 country. By the contraction of certain abdominal 

 muscles, the upper and lower walls of the abdomen 

 are drawn together to the extent, in the cockroach, of 

 one-eighth of the entire depth of the body, and a 

 compression from side to side takes place at the same 

 time. The tracheal tubes are thus compressed, and air 

 is forced out at the stigmata ; on the relaxation of the 

 muscles, the elasticity of the tracheal tubes themselves, 

 resulting from the coiled spiral thread in their interior, 

 then restores the body to its normal form, while air in 

 consequence enters at the stigmata. In the cockroach, 

 the thoracic segments have sufficient mobility to permit 

 of their taking some part in the movements of respira- 

 tion, even when the insect is at rest, in which respect 

 it differs from most other insects. In order, therefore, 

 clearly to realise how a cockroach breathes, we have to 

 bear in mind that, concurrently with the rise and fall 

 of the body walls, ten little jets of air alternately enter 

 at and issue from as many openings in the insect's 

 sides, the outward-tending jets of course carrying with 

 them" the carbonic acid and water vapour produced as 

 the insect discharges its vital functions. 



