THE RESPIRATORY ORGANS OF INSECTS. 375 



to constitute an occlusor apparatus, provided with a muscle, 

 by the contraction of which communication with the external 

 air can be cut off. This occlusor apparatus, long ago de- 

 scribed in certain insects by Strauss-Durckheim, Newport, 

 Burmeister, Siebold, and others, has recently been specially 

 investigated by Landois and Thelen, 1 who describe it as 

 usually consisting of four essential parts: the bow (Ver- 

 schlussbtigel),the lip (Verschlussband), the lever (Verschluss- 

 hebel), and the muscle. The bow is a thickening of one-half 

 of the circumference of the chitinous lining. The lip is formed 

 by the other half of the circumference, and the lever is a 

 chitinous process connected with one end of the bow, or with 

 the lip. When the lever is single, the muscle which is at- 

 tached to it passes over the lip and is inserted into the oppo- 

 site end of the bow. When it contracts, it therefore presses 

 the lip against the bow. When two levers are present, they 

 are attached to opposite ends of the lip and bow, and the 

 muscle extends between their extremities. The effect of its 

 contraction is to thrust the free edge of the lip against the 

 bow. 



The tracheal trunk which arises from a stigma may ramify 

 without communicating with the rest ; but, usually, the tracheae 

 which proceed from each stigma enter into more or less exten- 

 sive anastomoses. Very commonly the main trunks of each 

 side give off wide anastomotic branches, which unite and form 

 a longitudinal trunk on each side of the body, while transverse 

 trunks often connect the main tracheae of opposite sides. 



In many insects, especially those which possess great 

 powers of flight, more or fewer of the tracheae become dilated 

 into sacs, in which the spiral marking of the chitinous lining 

 is interrupted or disappears. In Bees and Flies, a vast air-sac 

 is thus developed, on each side of the abdomen, from the 

 longitudinal anastomotic trunk. 



The aquatic larvae of many Orthoptera (Ephemeridoe, 

 Agrion, Calopteryx) and Neuroptera, and of some Diptera, 

 Lepidoptera, and Coleoptera, though provided with a fully- 

 developed tracheal system, possess no stigmata. The somites 

 of the abdomen or of the thorax are, however, provided with 

 delicate foliaceous or filamentous processes, into which 

 branches of the tracheae enter and ramify. The air contained 

 in these tracheae is therefore separated from that dissolved in 



1 " Der StiamenverschluBS bei den Insecten." (ZeiUchrift fur wissen- 

 echaftliclve Zoologie, 1867.) 



