INSECTA 463 



editions may have accounted for the onset of metamorphosis, the pupal 

 stage being evolved for the purpose of surviving cold periods while 

 in a quiescent state. 



The most interesting fact, however, is that the main evolution of 

 our specialized bees, flies and butterflies coincided in point of time 

 with the evolution of the flowering plants to which by their manner 

 of feeding they are now on the whole so inseparably bound. 



Class APTERYGOTA 



Primitively wingless insects carrying on the abdomen a varying 

 number of paired appendages other than the external genitalia and 

 cerci. Metamorphosis slight or absent. 



Order THYSANURA (Bristle-tails) 



Biting mouth parts (Fig. 301); antennae many-jointed; compound 

 eyes present; abdomen of eleven segments, some or all of which bear 

 styliform appendages which probably represent the coxites of limbs 

 no longer present; anal cerci usually jointed, rarely (e.g. Japyx) in 

 the form of forceps. 



Lepisma saccharina (Fig. 321), the common "silver fish" which 

 inhabits dwellings of man, and Machilis (Petrobius) maritimuSy found 

 above high-tide mark along the sea shore and estuaries, are common 

 examples. In Machilis (Figs. 322, 301) interesting features are pre- 

 sented by the well-developed superlinguae and the jointed mandibles 

 both of which are primitive characters. The superlinguae in Machilis 

 are paired structures attached to the hypopharynx and possess inner 

 and outer lobes and a palp-like process. This superficial resemblance 

 to maxillae gave considerable weight to the view that an additional 

 head segment was involved. Embryological evidence in support of 

 this conclusion is of a doubtful nature, and the most acceptable view 

 to take is that the superlinguae are processes attached to the hypo- 

 pharynx and perhaps homologous with the paragnaths of Crustacea. 



Order COLLEMBOLA (Springtails) 



Small wingless insects with biting mouth parts deeply withdrawn 

 into the head ; compound eyes absent ; 6-segmented abdomen which 

 often carries three pairs of highly modified appendages serving the 

 purposes of adhesion and jumping; a tracheal system is commonly 

 absent and there are no Malpighian tubules; metamorphosis absent. 

 Four-jointed antennae, ocelli and postantennal sensory organs are 

 characteristic features of the head. 



