376 THE ANATOMY OF INYERTEBRATED ANIMALS. 



the water only by a very thin layer of integumentary tissue, 

 and an exchange of gaseous constituents between the two 

 readily takes place. These are often called branchiae, but 

 they are obviously of a totally different nature from true 

 branchiae. The larvae of some Dragon-flies (Libellula and 

 ^Jschna) present yet another form of respiratory organ. Al- 

 though they possess a pair of thoracic stigmata, these appear 

 to have little or no functional importance, but respiration is 

 effected by pumping water into and out of the rectum. The 

 walls of the latter are produced into six double series of 

 lamellae, in the interior of which tracheae are abundantly dis- 

 tributed, and which play the same part as the tracheal bran- 

 chiae just mentioned. These rectal respiratory organs, in 

 fact, appear to be a complicated form of the so-called "rectal 

 glands," which are so generally met with in insects. 



The chief agent of the movements of expiration and in- 

 spiration in insects is the abdomen, the capacity of which 

 may be diminished by the approximation of its terga and 

 sterna, and the shortening of its length by the retraction of 

 its posterior into its anterior somites ; while it may be en* 

 larged by movements in the opposite directions. When the 

 cavity is enlarged, air rushes in at the stigmata, and when it 

 is diminished, if the stigmata are open, expiration occurs ; 

 but, if the stigmata are shut, the effect of the expiratory act 

 must be to drive the air into the ultimate ramifications of the 

 tracheae. The movements of inspiration and expiration vary 

 in rapidity with the condition of the insect. In the Bee, 

 Newport observed that in the state of rest they were as few 

 as forty, but that they rose to one hundred and twenty with 

 muscular exertion. 



The air-sacs doubtless assist flight by the diminution of 

 the specific gravity of the insect, which follows upon their 

 distention. 



The sounds produced by insects 1 are, in a great propor- 

 tion of cases, effected by the friction of hard parts of the in- 

 tegument one against the other. Thus the Grasshopper rubs 

 the femur of the hind leg against a ridge on the anterior 

 wing, and the chirp of the Crickets and Locusts is produced 

 by the friction of the elytra. The parts which thus rub to- 

 gether are provided with serrations and ridges, which have a 

 constant and characteristic disposition. The longicorn Bee- 



1 See Landois, " Die Ton- und Stimm-Apparate der Insecten." (Zdtechrift 

 fwr wise. Zoologie, 1867.) 



