78 Second Stage of Insects. 
double arch, one above the other, as in our own verte- 
brate branch. This sac, looking like a miniature bag 
of grain, grows by absorption, becomes articulated, and by 
budding out is soon provided with the various members. 
As in higher animals, these changes are consequent upon 
heat, and usually, not always, upon the incorporation within 
the eggs of the sperm cells from the male, which enter 
the egg at an opening called the micropile. The time it takes 
the embryo inside the egg to develop is gauged by heat, 
and will, therefore, vary with the season and temperature, . 
though in different species it varies from days to months. 
The number of eggs which an insect may produce is sub- 
ject to wide variation. There may be a score; there may 
be thousands. ; 
THE LARVA OF INSECTS. 
From the egg comes the larva, also called grub, maggot, 
caterpillar, and very erroneously worm. These are.worm- 
shaped (Fig. 24), usually have strong jaws, simple eyes, 
and the body plainly marked into ring divisions. In some 
insects there are fourteen of these rings or segments, or ten 
beside the head and three rings of the thorax. In bees and 
nearly all other insects(Fig. 24, /) there is one less abdomi- 
nal ring. Often, as in case of some grubs, larval bees, and 
maggots, there are nolegs. In most grubs there are six legs, 
two to each of the three rings succeeding the head. Besides 
these, caterpillars have usually ten prop-legs farther back on 
the body, though a few—the loopers or measuring caterpil- 
lars—have only four or six, while the larve of the saw-flies 
have from twelve to sixteen of the false or prop-legs. The 
alimentary canal of larval insects is usually short, direct, 
and quite simple, while the sex-organs are slightly if at all 
developed. The larve of insects are voracious eaters— 
indeed, their only work seems to be to eat and grow fat. 
This rapid growth is well shown in the larva of the bee 
which increases during its brief period from egg to full 
grown larva—less than five days—from 1200 to 1500 times 
its weight. As the entire growth occurs at this stage, their 
gormandizing habits are the more excusable. I have often 
been astonished at the amount of food that the insects in 
