XI 



PHYLUM ARTHROPODA 



583 



it escapes from the egg differing from the adult only in its 

 smaller size, and in the absence of wings, which grow out sub- 

 sequently from the terga of the meso- and meta-thorax. Between 

 its hatching and its complete development the young Cockroach 

 undergoes no fewer than seven " moults " or changes of skin, in 

 which all the chitinous parts become thrown off and renewed. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Insecta are air-breathing Arthropoda, in which the body 

 consists of three well-marked regions head, thorax, and abdomen ; 

 the head devoid of external segmentation, bearing compound eyes, 

 a pair of antennas situated on the prostomium, but post-oral in 

 origin, mandibles, and two pairs of maxillae ; the thorax of three 

 segments each bearing a pair of legs, and the second and third 

 usually wings ; the abdomen composed of a varying number of 

 segments (7 11), which are devoid of appendages in the adult 

 condition. A liver is absent, but salivary glands are always 

 present. There is an elongated tubular heart, divided into eight 

 chambers, situated in the abdomen ; the vessels themselves are not 

 highly developed. The Insecta are, almost without exception, air- 

 breathers, and the organs of respiration take the form of branching 

 tubes, the tracheae, by means 

 of which air is conveyed to 

 all parts of the body. The 

 nervous system and sense- 

 organs are highly developed. 

 The excretory organs are a 

 number of blind tubes, the 

 Malpighian tubes, appended 

 to the intestine. The sexes 

 are separate ; development is 

 sometimes direct, more usually 

 complicated by a metamor- 

 phosis. 



ORDER 1. AFTER A (Collem- 

 bola and Thysanura). 



Insecta in which the wings 

 are absent, and the surface 

 is covered either with scales 



i - T-I , . FIG. 467.- Lepisma. (After Gu^rin 



or narrs. Eyes are sometimes Percheron.) 



absent ; sometimes there are 



groups of ocelli; sometimes compound eyes. The segments of 



the thorax are not fused together. Some progress by running 



