IV. INSECT A: HEX APOD A, APTERYGOTA. 477 



these groups are not considered natural and the attempt has been made to 

 divide them into more or fewer groups. Here the Pseudoneuroptera or 

 Aphaniptera are separated from the Neuroptera, the wingless forms or 

 Apterygota from the Orthoptera. 



Order I. Apterygota. 



At the bottom of the Hexapoda come forms which lack wings 

 and which show no evidence of having descended from winged an- 

 cestors. They are regarded as slightly modified descendants of 

 the ancestral Hexapod. Besides the lack of wings they show many 

 primitive characters; compound eyes are poorly developed or lack- 

 ing; the tracheal system, when not degenerate, consists of isolated 

 tracheal bushes, rarely connected by longitudinal trunks (fig. 

 479) ; the mouth parts, resembling somewhat those of Orthoptera, 

 are for biting, though frequently rudimentary; the development 

 is always ametabolous. 



Sub Order I. THYSANURA (Bristle-tails). Body 

 elongate, with long bristles (cerci) at the hinder end. 

 Lepisma saccharina,* silver fish, common among old 

 books and papers, does considerable damage. It is 

 covered with shining scales. Campodea * (fig. 400), 

 with rudimentary abdominal appendages. Machilis* 

 lapyx* with caudal forceps. 



Sub Order II. COLLEMBOLA (Spring-tails). Com- 

 pressed forms in which the bristles bent under the body 

 serve as a spring, throwing the animals (one to three 

 mm. long) forwards. Podura *; Anurida maritima* 

 in tide pools ; Entomobrya *; Lipura *; Achoreutes ni- 

 valis* the snow flea. 



Order II. Archiptera (Pseudoneuroptera). 

 These represent the primitive forms of winged 

 insects. The elongate body consists of numerous Packard'.) (After 

 segments and usually bears the cerci of the Thysanura. The 

 wings are delicate and transparent, supported by a close net- 

 work of nervures, both pairs being very closely alike. The 

 mouth parts are of the typical biting kind; the maxillae have 

 lacinia and galea; the labium, with glossa and paraglossa, is 

 frequently deeply cleft. These points of primitive structure 

 are correlated with a primitive, usually hemimetabolous de- 

 velopment. The distinction between larva and imago is largely 

 one of presence or absence of wings, although larval organs like 

 gills (Amphibiotica) may occur. Frequently the development is 

 direct when the imagines, as in some Termites and the Psocida?, 

 are wingless. 



