IMrORTANCE OF THE MOUTH PARTS. 49 



In tliose insects, on the other hand, which belong to 

 the second type, the mandibles and maxillae are not 

 articulated with the skull or otherwise connected with 

 it; but their bases are retracted within the cavity of the 

 shull, surrounded by muscles, whilst generally only their 

 points project outside the mouth. In this case the 

 appendages in question can be protruded and retracted, 

 but not moved laterall}^ against each other. They may 

 be used as pungent-instruments, but not for biting ; 

 and in this case the skull has generally much less con- 

 sistency than where the mandibles articulate with it." 

 .... " But, besides these two principal types, there 

 exists a third, which, as it were, connects them. In 

 some insects the mandibles and maxillae do not properly 

 articulate with the skull, and are retracted within the 

 cavity of the skull, so that only their apices are visible 

 outside the mouth ; but, nevertheless, they are calcu- 

 lated for biting, and capable of being moved laterally 

 against each other. The absence of articulation or 

 other connection 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 maxillas ; at the same time 

 it is to be observed that, in the case of this inter- 

 mediate type, the process of protrusion is effected by a 

 special contrivance, entirely different from that whereby 

 the mandibles and maxillge of the ordinary retracted 

 type are moved forwards and backwards. This inter- 

 mediate type, which reminds us of the arrangement of 

 the mouth of the Crustacea, is amongst insects only 

 found in Thysanuea (Collembola), and affords one of 

 the two principal characters of this order (or sub-order), 

 the other being this, that they remain in the larval stage 

 without undergoing any metamorphosis at all. Not 

 only are the individuals possessing perfectly developed 

 sexual organs without even a vestige of wings, but the 

 segmentum mediale forms a complete ring as in larvae, 

 and they are unguligrade ; their eyes, finally, when 

 they do occur, are single or, at utmost, agglomerated. 



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