14 
entirely valid, shows that they really represent the terminal sclerite of 
the metathorax, namely, the inetascutellum. This is plainly apparent 
from an examination of the genus Cephus, where the parts are very 
plainly differentiated and their relationshix) easily deciphered. There is 
a suture or fold separating the narrow anterior margin of this sclerite, 
but the portion so separated is intimately joined to the posterior por- 
tion and need not be separately designated. This sclerite is strongly 
incised at the apex centrally and this incision is covered with a white 
membrane, which, in descriptions, is commonly referred to as the 
white blotch of the so-called basal abdominal segment. The universal 
occurrence of this white blotch and its slight variation, except in the 
case of the larger groups of genera, make it ordinarily of little value in 
descriptions of species. In the comparisons of older descriptions the 
white spot on the basal segment will be understood to mean this 
blotch, and in harmonizing these with the characterizations of species 
in the following pages it must also be remembered, in referring by num- 
ber to the segments of the abdomen, that the so-called iirst segment 
belongs to the thorax. 
The pair of white spots occurring on the upper edge of the meta- 
scutum, termed cenchri, also occur uniformly in all Tenthredinidae and 
present no important variation in genera, and although they have been 
referred to in most of the older descriptions, it has not been deemed 
necessary to mention them m the characterizations of the following 
pages. These spots, uniformly oval and whitish in color and bearing 
a hexagonal surface sculpturing, have not been understood so far as 
their function is concerned. I am inclined to believe them to be sound 
organs, and that by the rubbing of the base of the subcostal veins of 
the hind wings over them a vibration of the cenchral plate or of the 
plate and vein results, which produces sounds audible to the insect ear. 
The structure of the cenchri has been hitherto erroneously given ; they 
consist uniformly of projecting plates attached basally, which x>rotect 
or cover openings into the thorax. In the case of the Lydime, the 
plate projects or is distinctly raised above the general surface, so 
that the free edge is plainly noticeable. The idea has therefore been 
that in the Lydime the true cenchri are covered by an overhanging 
plate. In this subfamily, however, these plates are the cenchri, there 
being no membrane or structure beneath them; and in other subfami- 
lies the posterior free edge fits down more closely into the opening of 
the cavity, so that the fact that it has a free posterior and lateral 
margin may only be discovered by dissection. 
The lateral and ventral aspect of the thorax includes, for the pro- 
thorax, an episternum and a central sternal plate; for the mesothorax, 
an epimeron and an episternum, and other sclerites which are rudi- 
mentary or unimportant. The meso-epinieron is very large and repre- 
sents the bulk of the side and venter of the thorax. The divided 
sclerite immediately back of it, which supports on its upper extremity 
