[Illustration: FIG. 135.--COMPOUND EYE OF INSECT. 1, hexagonal facets of crystalline cones. 6, blood vessel in optic nerve.] =Inherited Habit, or Instinct.=--Insects and other animals inherit from their parents their particular form of body and of organs which perform the different functions. For example, they inherit a nervous system with a structure similar to that of their parents, and hence with a tendency to repeat similar impulses and acts. Repeated acts constitute a habit, and _an inherited habit is called an instinct_. Moths, for example, are used to finding nectar in the night-blooming flowers, most of which are white. The habit of going to white flowers is transmitted in the structure of the nervous system; so we say that moths have an instinct to go to white objects; it is sometimes more obscurely expressed by saying they are attracted or drawn thereby. =Instincts are not Infallible.=--They are trustworthy in only one narrow set of conditions. Now that man makes many fires and lights at night, the instinct just mentioned often causes the death of the moth. The instinct to provide for offspring is necessary to the perpetuation of all but the simplest animals. The dirt dauber, or mud wasp, because of inherited habit, or instinct, makes the cell of the right size, lays the egg, and provides food for offspring that the mother will never see. It seals stung and semiparalyzed spiders in the cell with the egg. If you try the experiment of removing the food before the cell is closed, the insect will bring more spiders; if they are removed again, a third supply will be brought; but if taken out the third time, the mud wasp will usually close the cell without food, and when the egg hatches the grub will starve. =The Development of Insects.=--The growth and molting of the grasshopper from egg to adult has been studied. All insects do not develop exactly by this plan. Some hatch from the egg in a condition markedly different from the adult. The butterfly’s egg produces a wormlike caterpillar which has no resemblance to the butterfly. After it grows it forms an inclosing case in which it spends a quiet period of development and comes out a butterfly. This change from caterpillar to butterfly is called the _metamorphosis_. The life of an insect is divided into four stages: (1) _egg_, (2) _larva_, (3) _pupa_, and (4) _imago_, or perfect insect (Figs. 136, 137, 138). [Illustration: FIG. 136.--Measuring worm, the larva of a moth.] [Illustration: FIG. 137.--Pupa of a mosquito.] [Illustration: FIG. 138.--THE FOUR STAGES OF A BOTFLY, all enlarged. _a_, egg on hair of horse (bitten off and swallowed); _b_, larva; _c_, larva with hooks for holding to lining of stomach; _d_, pupal stage, passed in the earth; _e_, adult horse fly.]