HYMENOPTERA SESSILIVENTBES. 



757 



two ; larvae with thoracic legs and usually with pro-legs ; they live on plants. 

 Female furnished at the extremity of the body with saws or boring apparatus, 

 usually concealed. 



Fam. 1. Cephidae. Small, slender Insects with weak exoskeleton ; 

 dorsal sclerite between thorax and abdomen divided into two ; the pro- 

 thorax free and long ; the anterior tibia bears one spine. The larvae 

 of Stem-sawflies live in the stems of plants or on tender twigs. They 

 are white and rather maggot-like and their thoracic legs are but feebly 

 developed. The female bores a hole with her two saw-like ovipositors 

 and deposits her eggs in the stem or twig ; the larva when hatched eats its 

 way along the stalk. In this manner Cephus pygmaeus does much harm 

 to corn in Europe and C. integer destroys the younger shoots of the willow 

 in America. About 100 species are known, chiefly from the Mediterranean 

 area. 



Fam. 2. Oryssidae. Dorsal sclerite between thorax and abdomen 

 not divided, antennae arising below the eyes, their bases overhung by a 

 ridge. This family has but one genus Oryssus, with about twenty species 

 of which one, O. abietinus, is British. The abdomen is closely fused with 

 the thorax and has little or no independent movement. 



Fam. 3. Siricidae. Large and often brightly coloured insects. The 

 anterior lobe of the mesonotum is in contact with the mesoscutellum and 

 not separated by the intrusion of the lateral pieces ; the first abdominal 

 dorsal sclerite is divided. The female has a well developed boring appara- 



FlG. 488. Tremex columba, North America. A Imago, female ; B pupa, female, ventral 

 aspect ; C larva ; a imperfect legs ; D parasitic larva of Thalessa (p. 760). (B and D after 



Riley.) 



tus. There are about a hundred species of Siricidae and they are chiefly 

 found amongst the forests of the Northern Hemisphere. The females by 

 the aid of the powerful saw, which arises far forward on the ventral surface 



