444 ZOOLOGY. 



stylets for piercing and enclosed in a sheath formed by the 

 pedipalpi. The respiratory organs are tracheae or entirely 

 wanting. 



CLASS : INSECTA. 



Air-breathing Arthropoda which possess the power of 

 flight. The presence of wings distinguishes Insects not 

 only from other Arthropoda, but from all other Inverte- 

 brata, since no others are adapted to aerial locomotion. 

 There are, however, some insects in which wings are 

 absent; in some cases this is obviously due to the dis- 

 appearance of the organs, but in the order Aptera there is 

 no evidence that wings were ever developed ; wings there- 

 fore, though highly characteristic, are not absolutely 

 diagnostic. The body is divided into three regions head, 

 thorax, and abdomen ; the head shows no external segmen- 

 tation and bears a pair of compound eyes, a single pair of 

 antennae, a pair of mandibles, and two pairs of maxillae. 

 The thorax consists of three segments, each bearing a pair 

 of jointed unbranched legs, and the second and third 

 segments each bearing a pair of wings, which are out- 

 growths of the terga, not appendages. The abdomen is 

 distinctly segmented, the maximum number of segments 

 being 11, but in many cases only 7 to 10 can be dis- 

 tinguished; the abdominal segments bear no fully de- 

 veloped appendages, but at the posterior end processes 

 representing reduced appendages may be present. The 

 heart is elongated, tubular, divided into eight chambers 

 situated in the abdomen ; blood-vessels are not highly 

 developed. The organs of respiration are trachae, i.e. 

 much ramified tubes supported internally by a spiral 

 thread; the air enters by a number of small apertures 

 called stigmata, situated on the sides of the thorax and 

 abdomen. The nervous system consists of cerebral and 

 ventral pairs of ganglia, highly developed. A liver is 

 wanting, but salivary glands are always present ; the 

 excretory organs are a number of caecal tubes, the Mal- 

 pighian tubes, opening into the intestine. The sexes are 

 separate and the development is sometimes direct, more 

 usually remarkable for a very complete metamorphosia. 



