364 Dr. F. Meiiiert on the Campodeae, 



and are retracted within the cavity of the skull, so that only 

 their apices are visible outside the mouth ; but nevertheless 

 they are calculated for biting, and capable of being moved late- 

 rally against each other. The absence of articulation or other 

 connexion with the skull allows of their being to some extent 

 pushed out of the mouth when in use, which is neither possible 

 nor necessary in the case of free biting mandibles and maxillce ; 

 at the same time it is to be observed that, in the case of this 

 intermediate type, the process of protrusion is effected by a 

 special contrivance, entirely different from that whereby the 

 mandibles and maxillse of the ordinary retracted type are moved 

 forwards and backwards. 



This intermediate type, which reminds us of the arrangement 

 of the mouth in Crustacea, is amongst insects only found in 

 Thysanura, 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 undei'going any metamorphosis at all. 

 Not only are the individuals possessing perfectly developed 

 sexual organs without even a vestige of wings, but the seg- 

 mentum mediale forms a complete ring as in larvae, and they 

 are unguligrade*; their eyes, finally, when they do occur, are 

 single or at the utmost agglomerated. This larval character is 

 typical for these insects ; and we cannot fancy winged species 

 interspersed amongst them in the same way as apterous species 

 occasionally occur in the families of typically winged insects. 

 Thysanura may be regarded as forming two families, Cam- 

 podese and Podurse, of which for the present only the former 

 will occupy our attention. I do not consider that the Lepismse 

 properly belong to Thysanura, but class them with Ulonata, 

 amongst which they occupy the lowest place, being apterous 

 and unguligrade. In the construction of the labium and the 

 maxillae Lepisma agrees perfectly with Blatta ; but the mandibles 

 have this peculiarity — that although their whole external side is 

 free, so that they cannot be called retracted, they are but im- 

 perfectly articulated with the skull, being destitute of condyli, 

 and receive the necessary support during mastication from such 

 an intervening chitiuous piece as will be described hereafter in 

 Campodea. In Machilis both mandibles and maxillae are sup- 

 ported by such an inner piece. I admit that these and other 

 peculiarities in the anatomy of Machilis and Lepisma render it 

 difficult to draw the limit between Ulonata and Thysanura ; and 

 may be they will ultimately have to be united ; but at any rate 

 the two families of Campodese and Podurae will always stand 



* See, on this term. Prof. Schiodte's jiaper on Cerambyccs, translated 

 in the Ann. & Maa;. Nat. Hist. ser. 3. vol. xv. 



