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 of Hesperobium 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 thisrevision except the two marked 
by prefixed asterisks; these subtribes are distinguishable by 
the following characters: — 
Prosternum abbreviated between and under the ae forming an acute 
point which does not attain the mesosternum. Ct bpeseteasdecssene. FF 
Prosternum prolonged posteriorly in a more or san abule point which attains. 
the mesosternum, but not much dilated under the coxae..... fave evks 9: 
Prosternum attaining the mesoternum and also greatly dilated laterally 
under the coxae as far as the inflexed sides of the pronotum — hy- 
POMETA.< oc cccc cece cccccecccese secs cesses cece ee sees ceeees tees seus 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, ....e.ccccee vcccsscccccccsesccccce & 
Antennae posteriorly flexile, the basal joint less elongate and with a posterior 
apical sinus; anterior tarsi variable.... 1.22 secesccccescscsccccesecs 4 
