72 INJURIOUS AND USEFUL INSECTS 



where the corresponding segment is particularly large. These 

 thirteen segments are divisible into three thoracic and ten 

 abdominal. The last but one bears a short dorsal horn, 

 resembling that of a Sphinx-larva, though of smaller size. The 

 first segment behind the head (prothorax) is small, and looks 

 like a mere neck, the meso- and metathorax are large, not 

 clearly separated, and make a prominent hump. As usual in 

 Lepidopterous larvae, each thoracic segment bears pointed, 

 five -jointed legs, while pseudopods or false feet, unjointed 

 and ending in coronets of hooks, are borne upon the 3rd, 4th, 

 5th, 6th, and loth abdominal segments. Shortly before a 

 moult a peculiar dark ring appears in the prothorax; this is 

 the new larval head, which forms, not within the old one as 

 in most insects, but behind it. The soft structures are 

 gradually withdrawn into it, and before the skin is cast, 

 the old head is nothing but an empty shell. The same 

 thing happens in some other insects, with a relatively small 

 larval head ; it is to be explained by the lack of space within 

 the old head, and is a simple case of a method of develop- 

 ment which in certain Diptera becomes very complex and 

 peculiar. 



The internal structure of the larva is much the same as in 

 other caterpillars. The silk-forming organs have naturally 

 received much attention from insect-anatomists, and call for 

 some special notice here. In insect-larvae there may be several 

 successive pairs of salivary glands, which take different forms, 

 and pour out different secretions. In the silkworm one pair 

 of these glands are large and easily demonstrated by opening 

 the body from beneath. We then see the long, coiled, and 

 rather voluminous glands extending through a great part of 

 the length of the caterpillar ; they are so long that one of 

 them, when uncoiled, is nearly five times the length of the 

 larva (14-15 in.). The extremity of the gland is very long, 

 slender, and tortuous, forming a tangled mass, which overlies 

 the intestine. This opens into a wider portion, of about twice 

 the diameter of the preceding, which is bent twice upon itself, 

 and when thus bent occupies about half the length of the 

 body — i.e. the two middle fourths. In front it gradually 

 narrows to form the salivary duct. Careful dissection with 

 a good lens brings to light further details. Where the two 

 ducts fuse together there is a muscular enlargement, the 



