What insects are, and how they are classified 



There are some segmented animals that are not Arthropods : notably 

 the segmented worms (Phylum Annelida). These do not have the 

 skin hardened into an exoskeleton, and never have segmented append- 

 ages. In addition, there are considerable differences in the internal 

 structure. 



The Latin word insectum, and the Greek word entomos both mean 

 *cut-into', and so our words 'insect' and 'entomology' might have been 

 apphed to all Arthropods. By general agreement we restrict these names 

 to only one of the Classes into which the Arthropods may be divided. 

 There are seven Classes of Arthropods, one of which — the Class 

 Trilobita — is known only from fossils. Among living Arthropods the 

 principal Classes are the following : 



Insect A (insects). The segments of the body are arranged in three 

 groups, forming the head, thorax and abdomen (Figs. 14, 15). The head 

 has one pair oi antennae, or feelers, and three pairs of mouthparts: man- 

 dibles, maxillae and labium — the last is a single structure, formed by fusion 

 of a pair of appendages. The thorax often has three pairs of segmented 

 legs, one pair on each of its segments, but never more than three pairs. 

 The abdomen has no true segmented legs: a few primitive insects have 

 simple appendages on the abdominal segments, and many insects, 

 especially larvae, have unsegmented 'false legs' (prolegs or pseudopodia) 

 (Fig. 15). Adult insects often have either one or two pairs oi wings arising 

 from the thorax. 



Crustacea (crabs, lobsters, shrimps, etc.). Nearly all live in water 

 and breathe by gills. They have two pairs of antennae, and five or more 

 pairs of segmented legs. Other appendages may be modified for diflferent 

 purposes, and may be biramous, i.e. forked into two. The body may 

 have many segments all alike, with appendages, or may be arranged in 

 two divisions, the cephalothorax and the abdomen. 



Arachnida (spiders, scorpions, mites, ticks, king-crabs). These are 

 often confused with insects, but can generally be distinguished by having 

 four pairs of legs instead of three (young mites may have only three). 

 The segments of the body are generally grouped into two divisions only, 

 cephalothorax and abdomen. They have, no antennae, but in their place 

 have claws, or chelicerae. 



Myriapoda (millipedes, centipedes). The name means 'with ten- 

 thousand legs', and indeed these are animals with a great many seg- 

 ments, all alike, and each bearing one pair of legs ; in the millipedes the 

 segments are arranged in pairs, and so they appear to have two pairs of 

 legs per segment. 



In addition there are the Classes Tardigrada and Pentastomida 

 which are small, and little known except to specialists. The 



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