IV. IKSECTA: UEXAPODA, APTEP.TGOTA. 477 



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

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

 Aphaniptera are separated from the Neuroptera, tlie wingless forms or 

 Apterygota from the Ortlioptera. 



Order I. Apterygota. 



At the bottom of the llexapoda come forms which lack wini^s 

 and which show no evidence of liaving descended from wina:ed an- 

 cestors. They are regarded as slightly modified descendants of 

 the ancestral Ilexajiod. 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. 

 ■1-70) ; the mouth parts, resembling somewhat those of Ortlioptera, 

 are for biting, though frequently rudimentary; the development 

 is always ametabolous. 



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

 elongate, with long bristles (ceroi) at the hinder end. '\ / 



Lepisma saccliarina* silver fisli, common among old 

 Ijooks and papers, does considerable damage. It is 

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

 with rudimentary abdominal appendages. MachUis* 

 lajyyx,'* 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*; Amirida maritima* 

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

 valis * the snow flea. 



Order II. Archiptera (Pseudoneuroptera). 



These represent the primitive forms of winged ^^^i,Jhai7^u,y"'"[\- 

 insects. The elongate body consists of immerous p?rckard.) *'^''*'"'' 

 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 arc of the typical biting kind; the maxillaj have 

 lacinia and galea; the labium, with glossa and 2^ai'ag'lossa, 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 Psocidte, 

 are wingless. 



