378 ZOOLOGY 
family of Hymenoptera with short bodies ; the abdomen is sessile. 
The antennae vary a good deal, but are never elbowed; their 
Fic, 212.—<Athalia spin- 
arum (Turnip Saw- 
fly). After Curtis. 
1. Saw- fly, magnified, 
with lines to left show- 
ing natural size. 
2. Eggs in incision in 
leaf. 
3. Egg, natural size. 
4,5, 6. Caterpillars feed- 
ing on turnip leaf. 
7. Pupa case. 
8. Pupa. 
variations afford a basis of classification. The tibia of the fore- 
leg is provided with two spines. The ovipositor is short, and 
retracted when not in use; it consists of two saw-like processes 
protected by two valves. 
The females deposit their ova in punctures in the epidermis 
of plants; one egg is usually laid in each slit. At the same 
time a drop of fluid is secreted, which is said to cause a flow 
of sap to the injured part, and this sap is stated to be absorbed 
by the egg, which increases in size. Only one genus—WNematus 
—forms galls, and these are found in the leaves of the willow 
tree. Nematus resembles the bees and wasps in the fact that 
its unfertilised parthenogenetic eggs give rise to males. As a 
rule in the animal kingdom parthenogenetic eggs give rise to 
females. 
The larvae are very like caterpillars. Usually they are 
found in colonies, and are of a brown or greenish colour. They 
may be distinguished from the young of Lepidoptera, which 
never have more than five pairs of pro-legs, by possessing six, 
seven, or eight pairs. Their heads are also more globular, and 
the eyes more distinct. When at rest they usually curl round 
the posterior end of their body (Fig. 212) in the form of a note 
of interrogation. They are frequently to be found on the leaves 
of willows, limes, poplars, and conifers. They are said to 
emit an acid fluid from lateral pores on their body when 
irritated. 
