STRUCTURES IMPLICATED IN METAMORPHOSIS 



I I 



has expressed the opinion that less than four are never found in 

 that part. But after growth has proceeded even for a short time 

 the segments of the head become fused, as it were, in one mass, 

 and are no longer distinguishable. The body segments of many 

 larvae present very slight differences, and have, therefore, a general 

 likeness to each other; but the perfect insect is evidently fur- 

 nished with three sets of segments, those of the head, body or tho- 

 rax, and abdomen. The segments of the head of the larva are fused 

 together, but those of the thorax, or chest, and of the abdomen, 

 remain distinct. The head of the larva is readily distinguishable 



LARV^ OF A DIPTEROUS INSECT. 



from the rest of the body, but as there are no feet attached to 

 the segments of the thorax and abdomen in hymenopterous and 

 dipterous larvae (bees and flies), no satisfactory distinction can be 

 made between the parts of those insects. However, in most cater- 

 pillars and beetle grubs the presence of two classes of legs enables 

 the rings of the chest and abdomen to be numbered and recog- 

 nised. In its most simple condition, as seen in a great many 

 larvae, a segment is homogeneous throughout, and a transverse 

 depression along a line where the tissues are thin separates it 

 from the others. These separations of the segments are shown 

 in the representation of the legless lai-v^e of the dipterous in- 

 sect. In other instances there arc two arches in each of the 

 growing segments, one above and the other below, and a mem- 



