i METAMORPHOSES OF INSECTS 27 



number being usually nine on each side of the insect 

 a pair for the first segment of the thorax, and one for 

 each of the eight anterior segments of the abdomen. 

 Sometimes, as in the case of fly-maggots, there is 

 only a pair of spiracles, which are placed at the end 

 of the body or near it. These larvae embed them- 

 selves in their nutriment, and would be stifled were 

 they unable to draw in air through the exposed tip of 

 the tail. Spiracles are of many forms of structure. 

 Suffice it to say, they admirably guard the entrances 

 to the tracheae, and are capable of closing them, 

 preventing the intrusion of foul air and other im- 

 purities ; this also permits of the retention of large 

 quantities of air in the tubes. The spiracles of the 

 abdomen are always much smaller than those of the 

 thorax, and the posterior * ones, which were very im- 

 portant in the larva, are almost imperforate in the 

 later time of growth. The circumstance arises, prob- 

 ably, from the change that takes place as regards 

 the region of the body in which respiration is prin- 

 cipally carried on in the two stages of insects. In 

 the larva it takes place chiefly in the abdomen, in the 

 imago in the thorax. In larvae the tracheae are always 

 more diminutive than in perfect insects compared 

 with the size of the individual, and are smallest in 

 the footless larvae of the bee family, which live long 

 in closed cells. 



Some larvae and pupae that inhabit the water 

 breathe like fishes by means of branchiae, or gills, 

 respiring the air which is mechanically mixed with 

 the water. These branchiae are simply expansions 

 of the external surface of the body, and, as a rule, 



