THE Crabs show us a diversity of habits. The long-legged Spider crabs dress themselves up in costumes of sponge and seaweed, which they gather and stick on their backs and legs ‘
to make themselves inconspicuous against the vegetation of their environment. They will change their dresses to suit their surroundings !
The largest living arthropod is a spider crab, living in Japan; the legs outstretched may together measure so much as ten feet. Crabs, like the Velvet crabs, swim quite easily by means of the paddle-shaped terminal joints of their last legs. Then there are the land crabs of tropical countries, which have large gill chambers lined by a membrane, through which oxygen is absorbed—the chambers act as lungs. They always keep in touch with the sea, for their young are marine and the adults go down to the shore at the breeding season.
The wood lice are the only Crustacea adapted for a life spent entirely on land. Their general appearance is well known. Most of the relatives of the wood lice are marine and some are inhabitants of fresh water, like the common Asellus of our English ditches; others show an intermediate state between the marine and terrestrial forms, and occur in rock-pools and between tide limits, one living just above high-water mark and yet within reach of sea spray. Others are iish parasites, some of which are peculiar in that they transform themselves from young free-swimming males into parasitic females.
The brown Oniscus and blue Porcellio are the common garden forms, which live in moist situations under stones and wood. They show the beginnings of an air tube system comparable to that of insects—but, in other respects, the wood lice are far removed from insects. One of the wood lice is sometimes found as a guest in the nests of ants.
The sense of sight in Crustacea is well developed and they possess two kinds of eyes; these will be described when dealing with insects, since they are similar in the two groups. Like insects, too, Crustacea appear to appreciate differences or qualities of colour. For example, green light rays appear to attract water fleas more than other colour rays. They move towards light in preference to darkness; this is a practical measure, for they feed on minute floating plants which require light for growth and multiplication.
Some Crustacea have balancing organs in the form of pits lined with stiff hairs which rest on nerve cells. The Crayfish, for example, collects sand grains and places them in the pit in such a way that they rest on the hairs. As the Crayfish changes its position from the normal, the pressure of the sand
grains on the hairs varies, and a message is passed on to the nervous system and the necessary correction made. If iron filings are substituted for sand grains, the position of the Crayfish can be controlled by means of a magnet; the Crayfish will swim along quite happily on its back under these conditions.
CREATURES THAT CARRY THEIR OWN LAMPS OTHER Crustacea possess light organs; some exude luminous secretions; the Euphausids—transparent, shrimplike creatures living near the surface of the sea—actually possess little lamps complete with lens and reflector.
Enough has been said to show the great range of crustacean form and life in salt and fresh water and on land. In point of numbers, as well as in parts of their structure, they could be called the insects of the sea, and from the insects of the sea, we must turn to the vaster assemblage which are the insects of the land.