EXTERNAL ANATOMY OF INSECTS. 557 
which is with reason thought to be the representative of 
the secondary wing of the tetrapterous Orders. You 
may see this even in the common crane-fly (Z7pula), in 
which there is a real alula, connected by means of a la- 
teral process, terminating in ligament, with this supposed. 
scutellum. If you examine further the same insect, you 
will easily find what I regard as the true one in the bi- 
lobed piece which receives the dorsolum, situated be- 
tween the wings, and to the sides of which they are at- 
tached. In Aszlus, Tabanus, &c., this part is transverse, 
and only distinguished on each side by an oblique im- 
pressed line; in the Muscide it is square, and marked 
by a straight transverse one. 
4. Frenum*, This appendage to the scutellum and 
dorsolum varies considerably in the different Orders, and 
in many cases, as you will see, is a very important part, 
being the process by which the former is mostly con- 
nected with the elytra or upper wings. In the Coleg- 
ptera, the elytra of which are nearly stationary in flight, 
and therefore less require any counteraction to prevent 
their dislocation, this part is commonly merely a process 
or incrassation of the under margin of the scutellum, 
which towards the base of the dorsolum is dilated to 
form the socket for the elytra. Its use as a countercheck 
in this Order is best exemplified in the common water- 
beetle (Dytiscus marginalis). ‘This at the inner base 
of the elytra has a membranous fringed alula resembling 
those of Diptera ; to the lower fold of this the extremity 
of the frenum is attached, which forms a right angle with 
the scutellum, and the upper fold is attached to the base 
of the elytrum>. The object of this appendage is pro- 
* Pirates VIII. UX. XXVIIL 7. > PratE XXIII, Fic. 6. ”. 
