132 
COMPARATIVE ZOOLOGY. 
mature form. This moulting is a very remarkable opera¬ 
tion. How the Lobster can draw its legs from their cases 
without unjointing 
or splitting them 
w T as long a puz¬ 
zle. The flesh be¬ 
comes soft, and is 
drawn through the 
joints, the wounds 
thus caused quickly 
healing. The cast¬ 
off skeleton is a per¬ 
fect copy of the an¬ 
imal, retaining in 
their places the del¬ 
icate coverings of 
the eyes and anten¬ 
nae, and even the 
Fig. 98.—Diagram of an Insect: A, head bearing the lining membrane of 
eyes and antennae; B, prothorax, carrying the first ® 
pair of legs; C, mesothorax, carrying the second the Stomach with its 
pair of legs and first pair of wings; D, metathorax, , 
carrying the third pair of legs and second pair of teeth, 
wings; E, abdomen, with ovipositor, F; 1, coxa, or rpi v 
hip; 2, trochanter; 3, femur, or thigh; 4, tibia, or 11U111 J 
shank; 5, tarsus, or foot; 6, claw. q£ Insects differs 
from that of Crustaceans in consisting mainly of a horny 
substance called cliitin and in containing no lime. The 
head, thorax, and abdomen are distinct, and usually con¬ 
sist of fourteen visible segments—one for the head, three 
for the thorax (called prothorax , mesothorax , and metatho¬ 
rax ), and ten for the abdomen. The antennae, or feelers, 
legs, and wings, as well as hairs, spines, and scales, are ap¬ 
pendages of the skeleton. As Insects grow only during 
the larval, or caterpillar, state, moulting is confined to that 
period. These skeletons are epidermal, deposited in suc¬ 
cessive layers, from the inside, and are, therefore, capable 
of but slight enlargement when once formed. 
