HYMENOPTERA.— TENTHREDINIDA. 95 
what resemble leaves separated by a smaller rounded lobe with radi- 
ated coste. In Cephus, the external valves and the ovipositor (fig. 
71. 24.) are formed as in the typical species, except that the saws 
and their supports are not transversely striated: the same also occurs 
in Xyela *, in which the saws and supports are membranous, except 
down the central line of union, where they are corneous (fig. 71. 14.). 
Fach of these saws is attached by strong curved muscles, so that it is 
capable not only of being protruded from the extremity of the abdo- 
men to a certain distance, but also of being considerably deflexed (as 
in fig. 70. 15.) : moreover, each of the saws, instead of being, like a 
common saw, affixed to its back support, is capable of being pushed 
forwards and backwards, whilst the support remains fixed. In some 
genera, Lyda, &c. ( fig. ’71.10.), the base of the saws is greatly di- 
lated and elbowed. With this admirably contrived pair of instruments 
the female Saw-fly forms a slit in the stems or leaves of plants, the two 
saws acting upon the same spot with an alternate movement, one 
being protruded, whilst the other is drawn backwards, and vice versd. 
The action of these instruments, and their structure, has been 
minutely described by Reaumur (J/émoires, vol. v.; and by Blot, in 
the Mem. Soc. Linn. du Calvados, vol. i.). There is considerable 
difference in the mode in which these incisions are made: thus Hy- 
lotoma Rosz makes a simple series of slits, whilst H. Pagana makes 
two series close together. Each series consisting of numerous slits, 
but each slit contains only one egg. Some species, on the other 
hand, introduce their eggs by means of their saws into the edges of 
leaves (Nematus conjugatus, Dahlb.), and others beneath the longi- 
tudinal ribs of the leaves. A few, indeed, merely fasten their eggs 
upon the outer surface of the leaves (Nematus Grossularia, &c.) at- 
taching them together like a string of beads (Reaumur, vol. v. pl. 10. 
f. 8.), whilst a few place them in a mass on the surface of the leaf 
(ibid. pl. 11. f. 8, 9.). When the wounds thus formed are completed, 
an egg is deposited in each, passing down the channel or cavity 
formed by the internal union of the saws and their supports, by which 
it is conducted to its proper destination: the deposition of each egg 
is accompanied by a drop of frothy matter, which is supposed to pre- 
vent the closing of the wound. Moreover, the sides of the saws are 
* By Dalman and Curtis the ovipositor of this genus is represented as single. I 
have however ascertained, by dissection, that it consists of two plates inclosed 
between the two ordinary valves. 
