INTERNAL STRUCTURE. 



9 



" Articulomm tarsorum progressio numerica in methodo naturali 

 non admitfeencla.'"' It must, however, be admitted that Latreille 

 did not carry out this rule in his own practice, for, as Westwood 

 says (Classification, i, p. 301), the tarsal system of Olivier was 

 almost universally adopted, chiefly in consequence of Latreille 

 having employed it in his numerous works. The last joint of the 

 tarsus is called the onychium and bears the double or single claws ; 

 in tree- and plant-frequenting beetles (e. g. Collyris, certain species 

 of Stenus, and many Phytophaga) it is strongly bilobed. 



The abdomen is divided into segments, but with regard to its 

 composition there has been much difference of opinion, and great 

 difficulty has been caused by the conflicting ideas regarding the 

 number of segments which have been expressed by various authors; 

 five or six are usually visible on the uuder side (these being called 

 ventral segments), but if the elytra are removed seven, eight, or 

 nine will be seen on the upper side. This is due, as Dr. Sharp has 

 pointed out (Cambridge Natural History, vi, p. 186;, to two facts : 

 " 1, that the hind coxae have a great and complex development, so 

 that they conceal the true base of the venter, which, moreover, 

 remains membranous to a greater or less extent, and thus allows 

 much mobility, and at the same time a very accurate co-adaptation 

 between the hard parts of the venter and the metasternum [except 

 in the MalacodermidvE, where this coadaptation is wanting, or is 

 imperfect] ; 2, that the terminal segments are withdrawn into the 

 interior of the body, and are correspondingly much modified, the 

 modification being greater in the case of the ventral than in that 

 of the dorsal plates." In spite of the work of Verhoeff (Deutsche 

 Ent. Zeitschr. 1893-4, etc.), and others, the question of the real 

 number of dorsal and ventral plates cannot be regarded as settled, 

 and students should be careful to make plain to themselves the 

 nomenclature of the segments adopted by any author whom they 

 may be consulting : as some regard the last dorsal segment as the 

 eighth, while others take it as the seventh, it is better in descriptions 

 to speak of the last and penultimate joints. 



Internal Structure. 



Many of the older writers on insects, such as Burmeister, 

 Dufour, Newport, etc., paid considerable attention to the internal 

 structure and economy of insects, and, to judge by the way in 

 which their work and figures are used by recent authors, they must 

 have been in the main very acute observers. The best general 

 books on these matters seem to be Packard's Text-Book of 

 Entomology and Kolbe's ' Insektenkimde ' : the work of Dr. Sharp 

 in the Cambridge Manual of Natural History, Vols. Y and VI, 

 is also useful, and there is much that is valuable in Burmeister's 

 Manual of Entomology (1836), pp. 119-301. The writers on 

 particular points of structure etc. are legion, as may be seen by 

 examining the bibliography of any particular section. 



