554 EXTERNAL ANATOMY OF INSECTS. 
but not always, it is distinguished from the dorsolum by 
being more elevated: this is particularly conspicuous in 
the genus Later, in which it is a flat plate elevated 
from the dorsolum by a pedicel; in Sagra the latter part 
is horizontal, while the scautellwm is vertical: and even in 
cases where the distinction is not so striking, these parts 
are separated either by a line, or some difference in their 
sculpture and substance. In this Order this part varies 
greatly, and often in the same tribe or genus, both 
in size and shape; being sometimes very large*, and 
sometimes very minute; sometimes very long, and some- 
times very short; sometimes nearly round, at others 
square; now oval or ovate, heart-shaped, triangular, 
acuminate, intire, bifid, &c. In the Orthoptera, though 
less conspicuous, it still is present as a triangular eleva- 
tion of the middle of the posterior part of the dorsolum, 
with the vertex either pointing towards the head, as in 
Blatta, or towards the tail, as in Locusta’. In the 
Heteropterous section of the Hemiptera (which, in 
columns of Mandibulata and Haustellata, appear to bear 
the same reference to the Coleoptera, that the Hymeno- 
ptera do to the Diptera, and the Homopterous Hemi- 
ptera to the Orthoptcra), the part we are considering is 
mostly very large and conspicuous, quite distinct from the 
dorsolum, and in some (Seutellera) covering the whole 
abdomen, as well as the Hemelytra and the wings; it is 
most commonly, as in the Coleoptera, obtriangular‘, but 
in the last-mentioned genus it often approaches to a pen- 
tagonal shape. Though usually so striking a feature in 
this tribe, in the aquatic bugs (Gerrzs, &c.) it is covered 
@ In Macraspis Macleay it is often half as big as an elytrum. 
b Prate VIII. Fie. 12. #’. * Ibid, Fre. 20..%'. 
EE 
