378 



ZOOLOGY 



family of Hymenoptera with short bodies ; the abdomen is sessile. 

 The antennae vary a good deal, but are never elbowed ; their 



Fig. 212. — Atlutlia 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 — Nematus 

 — 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. 



