222 ENTOMOLOGY FOR MEDICAL OFFICERS 



a queen, or female parent ; (2) of males, whose existence is 

 transient and perhaps precarious ; (3) of workers, which are 

 sexually arrested females and may be all alike or may be of 

 more than one caste ; and (4) of eggs, larvae, and pupae. The 

 chief business of the workers is to house, provide for, and 

 nurse the developing larvae. The queen produces the eggs, 

 and the ultimate issue of the eggs is only to a certain extent 

 predetermined : males are supposed to be the predestined 

 issue of unfertilised eggs, while the destiny of the larvae 

 issuing from fertilised eggs — whether they shall be sexually 

 perfect females (queens) or sterile females (workers) — appears 

 to depend upon the amount and quality of the food adminis- 

 tered to the larva at a particular stage of its development. 

 All the individuals of a community — workers, females, and 

 males — may be alike, as with the social wasps; or they 

 may be unlike, as with the social bees ; or the workers 

 may not only be unlike either sex, but may be split into 

 structurally different castes inter se, as among many ants. 



From the economic standpoint the great order Hymeno- 

 ptera is of prime importance ; but the medical officer has to 

 regard chiefly the Aculeate Hymenoptera that may inflict 

 venomous wounds, and those species of parasitic and 

 predaceous Hymenoptera that prey upon Diptera. The 

 latter question is complicated by the facts that some of 

 these also prey upon spiders and upon other insects that 

 themselves are enemies of Diptera, and that some of the 

 Dipterous victims are not, to say the least, known to be 

 harmful. 



The Hymenoptera are arranged in two suborders, which 

 are named, according to the form of the abdomen, Sessili- 

 ventres and Petiolata. 



I. Sessiliventres. The abdomen is broadly connected 

 with the thorax, so that there is no " waist " ; the trochanter 

 of the legs is composed of two pieces ; the larva is caterpillar- 

 like, but can be distinguished from the Lepidopterous cater- 

 pillar by the fleshy abdominal legs, or pseudopods, being 

 more numerous, being always present on the 2nd abdominal 

 segment, and having no booklets ; sometimes the larva is 

 sluglike. The suborder includes, among other forms, the 

 Saw-flies {TenthredinidcB), whose larvte feed on plants and 



