Miscellaneous. 483 



Determination of the Directing Element in tJie Jaws of Insects. 

 By M. Joannes Chatin. 



In a previous series of researches, the results of which I have 

 submitted to the judgment of the Academy *, I have studied 

 the variations which the jaws can present, not only in their 

 ensemhle, but in the parts of which they consist, wheii studied in 

 boring-insects. For the purposes of such investigations these repre- 

 sent the fundamental types ; remaining always near to its initial 

 state, the jaw permits of the accurate recognition of the smallest 

 details as to the development, relations, &c. of its different parts. 



Highly instructive for all that relates to the descriptive anatomy 

 of the organ though the boring-insects (" insectes broyeurs ") are, 

 they become insufficient when it has to be considered from the point 

 of view of philosophic anatomy. 



Among other questions in this connexion there is one which 

 has been generally left in the shade. The theory of Savigny has 

 traced in its broad lines the series of transformations which the jaw 

 undergoes in order to adapt itself to the varying mode of life of 

 insects. But what is the role to be assigned to the different parts 

 of the jaw in its numerous functional adaptations ? Ought they to 

 play an equal part, or should one of the parts be pre-eminent? 

 should it reduce the others to the position of satellites or assert 

 itself as the centre of the curious processes which so strangely 

 metamorphose the organ ? 



After a minute analysis, the question cannot be elucidated except 

 by varying within wide limits the types on which it is proposed to 

 determine which is the directing element of the jaw. 



It is easily settled when one passes from the Borers to the Hymeno- 

 ptera. According to a too widely spread opinion, their jaw would 

 always take on the character which it presents in the Apidte, in 

 which its appearance differs profoundly from that in the Borers. 

 There is nothing so baseless as this generalization ; in reality the 

 form proper to the Borers reappears in several genera ( Vespa, Micro- 

 gaster, &c.). The base of the organ is formed by a submaxilla, 

 above which rises the maxilla, iiguring as the central limb of the 

 jaw ; on its upper part it displays three appendages, among which 

 the palp is far the most developed, the two others (galea and inter- 

 maxilla) appearing to be still secondary. 



In Gonatopus, Xyphidrina, and Bracon it is seen that the galea 

 increases progressively, at the same time that the intermaxilla 

 approaches it more and more closely. 



In the genus Perilampus the formation of a mixed galoo-intor- 

 maxillary plate may be seen. This is definitely constituted in 

 Cepluis and Megacliile, much elongated, absorbing, so to speak, the 

 intermaxilla ; the galea is transformed into a powerful blade, which 

 becomes henceforth the principal part of the jaw. 



Here, then, is an organ completely modified, in no way resembling 

 what it was in the Borers, tending, on the contrary, to the form 

 presented by the Suckers, such as the Lepidoptera &c. 



I could pass at once to the examination of these latter, but it 



* ' Comptes Eeudus,' 1879-1887, and ' Concours pour le Grand Prix 

 des Sciences physiiques,' 1885. 



