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 ot 
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 bod 
on which the larva is placed. From the contractile nature ot 
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 Réaumur’s “Mémoire sur les Différentes Parties des Che- 

