13 



tbe fifth pair of spiracles, whcu present 2nd pair abdominal 

 legs ; ninth, the sixth pair of spiracles, when present 3rd 

 pair abdominal legs; tenth, the seventh pair of spiracles, 

 ■when present 4th abdominal legs ; eleventh, the eighth 

 pair of spiracles ; twelfth, the ninth pair of spiracles ; 

 thirteenth, the anal legs, these are always present. 



These joints, or segments, are covered by a skin con- 

 taining a peculiar principle, called chitine ; this skin serves 

 as a skeleton for the attachment of muscles, the same as our 

 osseous structures, or the bones of birds, but with this 

 exception, the bones which form our skeleton, and those of 

 birds, &c., are always internal, but with larvae they are 

 always external ; that is to say, this tegument, which has 

 more or less density, takes the place of bone — it is there- 

 fore called the dermoid skeleton. The density of the skin is 

 greater in the centre of each segment, where it joins to its 

 neighbour, and is very soft and pliant ; thus giving great 

 freedom of motion to the animal. The skin is sometimes 

 naked, at others thinly or thickly studded with long or 

 short hairs. The skin is very little elastic, so that upon the 

 internal growth of the larva, it becomes highly compressed, 

 and were it not for the beautiful contrivance of nature, 

 the little animal would be suflFocated by its own growth. 

 To avoid this, when a sufficient degree of tension is given 

 to the skin, the animal becomes sluggish, refuses to eat, 

 and in about thirty hours the skin splits down the back, and 

 the larva comes out with a bright and more beautiful 

 covering. It now rapidly increases ia size, feed* 

 voraciously, and in the course of six or seven days becomei 

 80 compressed again in its straightened tunic, that it ceases 

 to feed, and the skin again bursts asunder. There are 

 generally from three to six of these dermoid, or skin 

 changes during the period of larval existence. 



To render clearer the changes we are about to explain, it 

 is very essential that we first take a brief survey of the 

 general organs of the larva as we find them during the 

 period of its existence. We will therefore examine them 

 in the following order: — Locomotion, vision, mastication, 



