THE ORGANS OF RESPIRATION. 165 



we call to mind also the general law which makes the insect body to 

 consist of thirteen segments, whereof one forms the head, three the 

 thorax, and nine belong to the abdomen, the number of the spiracles 

 is readily ascertained. The thirteen segments have namely twelve 

 connecting membranes, of which the first only (between the head and 

 pro-thorax) and the last are never supplied with spiracles, consequently 

 there cannot be more than ten on each side at most. But as the 

 number of the abdominal segments considerably varies, it consequently 

 frequently happens that there are fewer spiracles. I have observed 

 twenty in the water-beetles (Dyticvs)'. According to Degecr and La- 

 treille *, the locusts and Lepidoptera display as many : the Lamelli- 

 corniaand Cerambycina possess eighteen. Many Orthoptera, the Ter- 

 mites, and Libellulce possess the same number. The Hymenoptera have 

 but seven distinct abdominal segments, the last of which, according to 

 the general rule, bears no spiracle ; in general they possess sixteen : 

 Panorpa has fourteen ; many Diptera still fewer, as but five or six 

 distinct abdominal segments are perceived in them. 



125. 



II. The AIR TUBES are absolutely nothing but elongated spiracles, 

 although they are not always found, where the spiracles are placed. 

 They are only observed in insects which live in the water, namely, in 

 the larvae of many Diptera and some water-bugs (Nepa, Rannlra}, 

 and are placed either at the first or the last abdominal segment. They 

 here appear as either long or short horny tubes, which pass directly 

 from the general integument of the body, being open at the end, and 

 within the orifice they are surrounded by simple or plumose setae, or 

 else entirely unprovided with them. 



The larva of the common gnat (Culex, PI. III. f. 3) is very gene- 

 rally known as possessing this organ, which is placed obliquely at the 

 last abdominal segment. Simple branches of the tracheae pass into 

 this tube, opening where it terminates. The end of the tube is 

 surrounded by setae, and these support the animal upon the sur- 

 face of the water when it places itself there to breathe. In the pupa 

 state the tube at the end of the abdomen disappears, and instead of it 

 two bent tubes project from the thorax between the pro- and meso- 



* P. A. Latreille sur quelqucs Appendices du Thorax dcs divers lusectes. In Mem. du 

 Museum d'Hist. Nutmvllc, turn. vii. 



