30 INTRODUCTION. 



but " are retracted witliin the cavity of the skull, so 

 that only then' apices are visible outside the mouth ; 

 but nevertheless they are calculated for biting, capable 

 of being moved laterally against each other," and 

 differ, therefore, essentially from those of the suctorial 

 groups. " This intermediate type is amongst insects 

 only found in the Thtsanura, and affords one of the 

 two principal characters of this order or suborder, the 

 other being this, that they remain in the larval stage 

 without undergoing any metamorphosis at all." He 

 excludes the Le^nsmidce, therefore, from the Thysanura, 

 although the articulation of the mandibles approximates 

 to that of Podura. 



Lastly, in 1869, M. Tullberg published a valuable 

 monograph of the Lipuridce, in which family he includes 

 AcJwrutes. For reasons which will be given presently, 

 I am unable to agree with him in this. 



Having thus very briefly referred to the principal 

 memoirs which have appeared on the Thysanura and 

 CoUemhoIa, I will now proceed to consider the arrange- 

 ment and position of these two groups. 



