506 EXTERNAL ANATOMY OF INSECTS. 
of which they are stationed*; but many other genera 
of that Order are without them; as Myrmeleon, Asca- 
laphus, Hemerobius, &c. The Trichoptera and Lepi- 
doptera universally have them; though in the latter, 
except in Castnia and the Sphingide, they are not ea- 
sily seen. In the Hymenoptera they are usually very 
conspicuous, but in Larra and Lyrops, two genera of 
this Order, the posterior pair are scarcely discernible ; 
and in the neuter ants they are quite obsolete. In the 
Diptera, though many genera are furnished with them, 
yet many also want them; amongst the rest Latreille’s 
Tipularie, and all the horse-flies (Tabanus L.). The 
Pupipara usually have none; but in Ornzthomyia avicu- 
laria, one of that tribe, though extremely minute they 
are visible, arranged in a triangle, in the polished space 
of the vertex. 
As to the Number of the stemmata, three appears to 
be most universal. Reaumur mentions an instance in 
which he counted four in a fly with two threads at its 
tail; but great doubt rests upon this statement’. Some 
orthopterous genera, as Gryllotalpa, and many hemi- 
pterous, as Sczéellera, Pentatoma, Reduvius:, Cercopis, 
Fulgora’, &c., have no more than ¢wo; and in Larra 
and its affinities, as just observed, the posterior ones are 
obsolete, so as to leave only one discernible. 
Where there are three of these organs, they are usu~ 
ally arranged in an obverse ¢rzangle in the space behind 
the antenne, at a greater or less distance from them. 
a Prate VI. Fic. 10.1. 
> Reaum. iv. 243. He refers for this insect to plate xiv. without 
adding any number for the figure ; but no such is in that plate. 
© Prare XXVI. Fie. 40. i. 
4 Cercopis, Ibid. Fic. 42; and Fulgora, Fic. 41.1. 
