a Famili/ of ThysaunvR. 363 



they are adapted for sacking, this process is carried on either 

 by means of the lingua, as in bees, or by means of an extension 

 of the oesophagus, as in Glossata. Where this arrangement of 

 the mouth is adopted, the skull must possess a certain degree 

 of firmness in order to aiford sufficient support for muscles and 

 articulations. 



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

 second type, the mandibles and maxillai 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 maxillae, and is entirely analogous to 

 that of Piezata. 



Free mandibles and maxillre, 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 maxillas, of which neither pair articulates with 

 the skull. 



If next we take into consideration the larvse 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 larvse 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- 

 })hosis 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 maxillse do not articulate properly with the skull. 



