506 NATURAL HISTORY OF ARTHROPODS. 



a double chitinous membrane stretched between the stigma and the lower half of the 

 cup. It is by the vibration of these parts that the well-known hum is produced. 



The Hymenoptera are divided into two sub-orders, the Terebrantia or borers, and 

 the Aculeata or stingers. In the Terebrantia the caudal end of the abdomen is fur- 

 nished with an instrument employed as a saw or borer for depositing the eggs. In 

 the Aculeata the end of the body is furnished with a sting which is connected with a 

 poison gland. The antennae of the males usually have thirteen segments, and those of 

 the females twelve. The Terebrantia are lower in rank, both structurally and in intel- 

 ligence, than the Aculeata. Gerstacker has pointed out an important structural dif- 

 ference between the plant-eating Hymenoptera and the higher and more typical forms. 

 In the latter, or true Hymenoptera, the first abdominal segment of the larva, during 

 development, becomes transferred to the thorax, so that in the abdomen of the adult 

 we can never distinguish more than eight dorsal half-segments. In the phytophagoua 

 Hymenoptera this segment is retained in the abdomen, in which nine segments are 

 recognizable in the adult. 



Stjb-Oedee I. — Teebbeantia. 



In following out the plan of this work, it is necessary to discuss the Terebrantia 

 before we study the Aculeata. On some accounts this is undesirable, as the Aculeata 

 consist of the more typical forms, while the lower members of the Terebrantia are 

 anomalous both in structure and habits. Therefore, if the reader be not on his guard, 

 he will at first get a wrong impression respecting the characters of this order. 



The Terebrantia has been divided into three sections, the Phytophaga or plant- 

 eaters, the Gallicola or gall-inhabiters, and the Entomophaga or insect-eaters. To the 

 first section belong two families, the Tenthredinidse and Uroceridse. • The second sec- 

 tion consists of a single family, the Cynipidte. And to the 

 third section five families belong, the ChalcididsB, Procto- 

 l^l^U^ trupidas, Braconidffi, Ichneumonidse, and Evaniidge. 



The Tejstthredinid^ includes the insects popularly known 

 as saw-flies. They are of great interest to the systematist on 

 account of their aberrant characters. They show some strik- 

 ing affinities to the Lepidoptera. In the adult the base of 

 the abdomen is joined broadly to the thorax as in Lepidop- 

 tera, instead of being constricted so as to form a waist as in 

 the more typical Hymenoptera. The head is broad and the 

 thorax wide, closely resembling that of Lepidoptera. The 

 larvae are like caterpillars both in appearance and habits. 

 Like them, they have a more or less cylindrical body fur- 

 FiG. 629. — Larva of Zyda. nished with six true legs and a number of prolegs, and they 

 also feed on the leaves of plants. The larvae of saw-flies 

 have, however (except in the genus JLyda), from twelve to sixteen prolegs; while 

 caterpillars never have more than ten. 



When full grown, the larvae make parchment-like silken cocoons. Sometimes 

 these are attached to the plant upon which the larvas have been feeding, but more 

 often the larvae descend to or beneath the surface of the ground before transforming. 

 When the adult insect emerges it lays its eggs upon the plant upon which its larvae 

 are to feed. Sometimes these eggs are simply placed upon the surface of the plant ; 



