a Family of Thysanura. 363 
they are adapted for sucking, this process is carried on either 
by means of the lingua, as in bees, or by means of an extension 
of the cesophagus, as in Glossata. Where this arrangement of 
the mouth is adopted, the skull must possess a certain degree 
of firmness in order to afford sufficient support for muscles and 
articulations. 
In those insects, on the other hand, which belong to the 
second type, the mandibles and maxille are not articulated with 
the skull or otherwise connected with it; but their bases are 
retracted within the cavity of the skull, 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 laterally against each other. They 
may be used as pungent-instruments, but not for biting; and 
in this case the skull has generally much less consistency than 
where the mandibles articulate with it. 
In both divisions the labium (that is, the third pair of appen- 
dages of the mouth) plays about the same part, covering as it 
does, more or less, the other parts of the mouth from beneath. 
In the latter division it is generally prolonged into a tube, 
which often serves as a sucking-instrument, whilst the proboscis 
of Glossata, which formerly were classed with the insects of this 
division, is formed by the maxillz, and is entirely analogous to 
that of Piezata. 
Free mandibles and maxille, of which at least the former 
articulate with the skull, are characteristic of Eleutherata, 
Ulonata (Synistata), Glossata, and Piezata; whilst, amongst 
the larger orders, only Antliata and Rhynchota have retracted 
mandibles and maxille, of which neither pair articulates with 
-the skull. 
If next we take into consideration the larve of Insecta with 
perfect metamorphosis, we find amongst them two correspond- 
ing principal types, one of which is distinguished by the posses- 
sion of a well-developed skull, with which the mandibles articu- 
late, whilst those of the other type have no skull, properly 
speaking, and then the mandibles are retracted into the mouth 
without articulation. Generally speaking, the larve and the 
imagos of the same insects agree in this respect ; nevertheless 
exceptions are met with amongst Antliata, not a few Nemocera 
possessing in their larval state a well-developed skull and arti- 
culating mandibles. In insects which have no true metamor- 
phosis the construction of the mouth must necessarily remain 
the same in all the stages of development. 
But, besides these two principal types, there exists a third, 
which as it were connects them. In some insects the man- 
dibles and maxille do not articulate properly with the skull, 
