142 THE INSECT WORLD. 



constitute the thorax ; the other segments constitute the abdomen. 

 The head is formed of two scaly parts. It is often very deeply 

 hollowed out on its upper side, and divided into two lobes, which 

 contain in the angle formed by their separation the different parts 

 of the mouth. The head is uniform, rarely having, as far as our 

 caterpillars are concerned, any protuberance ; but in the tropi- 

 cal species it is often armed with prickles, spikes, and extraordinary 

 appendages. They are provided with six small simple eyes ; 

 isolated from each other. The mouth is armed laterally with 

 a pair of very solid horny mandibles, articulated by means of 

 vigorous muscles, and moving horizontally. It is the function 

 of the mandibles, as with the jaws, to divide the creature's food. 

 On the middle of a broad under-lip, one may perceive a little 

 elongated tubular organ, pierced with a microscopic orifice. This 

 organ is the spinning apparatus, which the animal uses in fabri- 

 cating the threads which it will one day require. It is a tube 

 composed of longitudinal fibres. It presents only one orifice, cut 

 obliquely, and capable of applying itself exactly to the body 

 on which the larva is placed. From the contractile nature of 

 this organ and the form of its orifice, combined with the 

 faculty the insect possesses of moving it in all directions, result 

 the great differences we observe in the diameter and form of the 

 threads. 



The external organs of the trunk and abdomen are the legs, 

 the spiracles, and various occasional appendages. The legs are 



Fig. 94. Scaly legs of the Oak and Elm Caterpillar. 



of two different kinds. The one, to the number of six, attached 

 by pairs to the trunk, are covered with a shiny cartilage, and 

 armed with hooks. These are the true legs. Fig. 94 represents, 

 after Reaumur's "Memoire sur les Differentes Parties des Che- 



