4 THE ORDERS OF INSECTS. 



characters of the three first classes, as the Insects are separated 

 from them by a great number of characters. 



Insects have six legs in the perfect state, and no more ; four 

 (or two) wings, two eyes, composed of a great number of facets, 

 and sometimes one, two, or three eyes of another kind, called 

 simple eyes, or stemmata, on the tip of the head. Their body 

 is composed of thirteen segments, divided into head, thorax, 

 and abdomen. They have neither heart, brain, nor nervous 

 system at all resembling those of the higher animals. The 

 place of the heart is supplied by an organ called the great 

 dorsal vessel, lying along the back ; and the brain and nervous 

 system of vertebrate animals are represented by a double row of 

 connected ganglia, or knots of nervous matter, lying along the 

 lower surface of the body. They breathe by means of spiracles, 

 or air-holes, opening on each side of the greater number of the 

 segments of the body. The muscular system is highly developed, 

 the muscles being far more numerous than those of vertebrate 

 animals. Insects pass through four stages, called respectively 

 egg, larva (or caterpillar), pupa (or chrysalis), and imago, though 

 these are more sharply defined in some insects than in others. 

 They also moult their skins more or less frequently in the larva 

 state ; and although they sometimes possess more than six legs 

 in this state (and occasionally none at all), yet the larvae of 

 insects are not generally liable to be mistaken for any other 

 animals. 



We have spoken of the thirteen segments of which the body 

 of an insect is composed ; the first forms the head, the second 

 to the fourth the thorax, and the remainder the abdomen. These 

 thirteen segments (except occasionally one or two of the terminal 

 segments) are generally distinct in larvae, but become more or 

 less welded together in the perfect insect, in which, however, 

 the three divisions of head, thorax, and abdomen are always dis- 

 tinctly visible, and are much more clearly defined than in the 

 larva state. 



The head contains the organs of sense, and the mouth. In 

 addition to the eyes, there is always a pair of long jointed 

 organs called antennae, which appear to be organs of touch, 

 smell, and probably of hearing. After the loss of these organs, 

 an insect becomes wholly incapable of directing its flight. The 

 antennae differ very much in shape in different insects, and are 

 called simple, pectinated, lamellated, clavate, etc., accordingly. 

 Sometimes they are straight ; sometimes sharply angulated in 

 the middle ; sometimes smooth ; sometimes hairy ; and often 

 furnished with long projections, giving them the- appearance of a 



