Casey — A Revision of the American Paederini. 19 



separated and may be most approximate at base or near the 

 anterior part of the post-oral surface of the head, with all 

 intermediate modifications. When considered broadly the 

 gular sutures undoubtedly form very valuable and reliable 

 generic characters in the Paederini. Such other characters 

 as have been found useful in the definition of genera and 

 species will be mentioned under the more or less detailed 

 descriptions or subjoined notes. 



Many of the Paederini have been found in the company of 

 ants of various species and it is quite possible that most of 

 them may be thus associated to a greater or less degree of 

 intimacy at some stage of their existence, but in only a very 

 few cases does this association assume the form of true and 

 life-long symbiosis, as in the case of Megastilicus for example. 

 A correspondent — Dr. W. M. Wheeler — recently sent me 

 a specimen oi Hesperohium flavicorne , with the statement that 

 it had been hatched from pupae found in the nests of Formica 

 incerta Emery. Probably in such cases the Staphylinid is an 

 intruder, for the subsequent life-history of the numerous 

 species of Cryptobia and Lathrobia does not appear to be 

 identified in any particular or peculiar way with that of the 

 ants and they have no specialized structures, as is virtually 

 always the case with true ant-guests. 



The tribe Paederini is composed as far as known to me at 

 present of twelve subtribal groups, all of which occur within 

 the geographical limits of this revision except the two marked 

 by prefixed asterisks ; these subtribes are distinguishable by 

 the following characters : — 



Prosternum abbreviated between and under the coxae, forming an acute 

 point which does not attain the mesosternum 2 



Prosternum prolonged posteriorly in a more or less acute point which attains 

 the mesosternum, but not much dilated under the coxae 9 



Prosternum attaining the mesoternum and also greatly dilated laterally 

 under the coxae as far as the inflaxed sides of the pronotum — hy- 

 pomera 10 



2 — Antennae anteriorly flexile and strongly geniculate, the basal joint 

 usually very much elongated and having an anterior sinus in its apical 

 margin ; anterior tarsi not dilated 3 



Antennae posteriorly flexile, the basal joint less elongate and with a posterior 

 apical sinus ; anterior tarsi variable 4 



