Elementary entomology ([c1912]) Elementary entomology elementaryentomo00sand Year: [c1912] INSECTS AND THEIR NEAR RELATIVES these four classes are based largely upon the manner in which the different segments are grouped together to form compact and distinct parts of the body, and by the number and position of the appendages. The Crustacea include the lobsters, crabs, crayfish, shrimps, bar- nacles, sow-bugs, etc., and are primarily distinguished from all other arthropods by the fact that they breathe by means of gills and live either in the water or in damp places. The body is divided into t
Elementary entomology ([c1912]) Elementary entomology elementaryentomo00sand Year: [c1912] INSECTS AND THEIR NEAR RELATIVES these four classes are based largely upon the manner in which the different segments are grouped together to form compact and distinct parts of the body, and by the number and position of the appendages. The Crustacea include the lobsters, crabs, crayfish, shrimps, bar- nacles, sow-bugs, etc., and are primarily distinguished from all other arthropods by the fact that they breathe by means of gills and live either in the water or in damp places. The body is divided into two main regions, the anterior segments be- ing usually covered by a single large shell forming the head-tho- rax, or cephalothorax, while the remaining segments form the ab- domen. Each segment usually bears a pair of appendages. On the head are found two pairs of antennae, and on the thorax and ab- domen are numerous FIG. 4. Sow-bug, or pill-bug (Porcellis laevis} appendages fitted for Enlarged. (After Jordan and Heath) walking or swimming. The only crustacean commonly mistaken for an insect is the little sow-bug, or pill-bug, found in greenhouses, under boards, or in damp places (Fig. 4). These rarely do any damage and may be readily distinguished from insects by the two pairs of antennae and the numerous appendages. The gills are to be found under plates on the lower side of the abdomen. The Arachnida include the spiders, scorpions, ticks, and mites, and are almost entirely terrestrial. The body is divided into the cephalothorax and abdomen, as in the Crustacea, but there are no antennae and but four pairs of legs. Although ticks and mites are not insects, yet they are so nearly related, and their inj uries to plants and animals are so similar to insect depredations, that they are com- monly included in economic entomology. Spiders are, if anything,
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