364 Dr. F. Meinert on the Campodee, 
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 maxille ; 
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 maxillz 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 undergoing 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 larve, 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- 
pode and Podure, of which for the present only the former 
will occupy our attention. I do not consider that the Lepisme 
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 
maxille 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 chitinous piece as will be described hereafter in 
Campodea. 1n Machilis both mandibles and maxille 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 Campodeze and Podure will always stand 
* See, on this term, Prof. Schiddte’s paper on Cerambyces, translated 
in the Ann. & Mag. Nat. Hist. ser. 3. vol. xv. 
