INSECTS
passengers on ships, they have spread to all countries where ships have gone.
Besides the household roaches, there are great numbers of species that live out of doors, especially in warm and tropical regions. Most of these are plain brown of various shades, or blackish, but some are green, and a few are spotted, banded, or striped. Different species vary much in size, some of the largest reaching a length of four inches, measured to the tips of the folded wings, while the smallest are no longer than three thirty-seconds of an inch in length. They nearly all have the familiar flattened form, with the head bent down beneath the front part of the body, and the long, slender antennae projecting forward. Most species have wings which they keep closely folded over the back. In the Oriental roach, the wings of the female are very short (Fig. 49 D), a character which gives them such a different appearance from the males (E) that the two sexes were formerly supposed to be different species.
The roach, of course, was not designed to be a household insect, and it lived out of doors for ages before man constructed dwellings, but it happens that its instincts and its form of body particularly adapt it to a life in houses. Its keen sense, its agility, its nocturnal habits, its omnivorous appetite, and its flattened shape are all qualities very fitting for success as a domestic pest.
Many kinds of roaches give birth to living young; but most of our common species lay eggs, which they inclose in hard-shelled capsules. The material of the capsule is a tough but flexible substance resembling horn, and is produced as a secretion by a special gland in the body of the female opening into the egg duct. The capsule is formed in the egg duct, and the eggs are discharged into it while the case is held in the orifice of the duct. When the receptacle is full its open edge is closed, and the eggs are thus tightly sealed within it. The sealed border is finely notched, and transverse impressions on the surface of the capsule indicate the position of the eggs within it.
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