STRUCTURES IMPLICATED IN METAMORPHOSIS. 
1 1 
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.E 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 larvae of the dipterous in- 
sect. In other instances there are two arches in each of the 
growing segments, one above and the other below, and a mem- 
