6 HYMENOPTERA ACULEATA. 



lies along the back of the abdomen, from wliiob tbe blood 

 is circulated through the vessels by the contractions and 

 dilatations of the organ. 



The Besinratury System is carried on through the agency 

 of spiracles, which open into the air tubes or tracheee, which 

 distribute the air through the system. These tracheae con- 

 sist of two main branches dilating towards the base of the 

 abdomeu into two very large vessels, which help the insect 

 to sustain itself in the air. 



The spiracles in the Aculeata are not easy to see, as those 

 of the abdomen are mostly at the base of the segments 

 liidden by the overlapping of the preceding one. That on 

 the first abdominal segment is however exposed, and those 

 of the metathorax and propodeum may be easily examined. 



l\p,2ii'oductive System. — In the J' the organs of this system 

 are, as in other insects, the testes, the spermatic ducts which 

 lead from the testes into the seminal vessels, and the 

 ejaculatory duct terminating in the intromittent organ. In 

 some cases the testes are enclosed in a common sac ; in 

 others distinct. In some the speimatic ducts lead directly 

 from the testes into the seminal vessels ; in others the 

 seminal vessels are at the ends of tubes which lead into the 

 spermatic duct. In the ? the organs are the ovaries, each 

 of which consists of a series of tubes united together at the 

 ends, varying considerably in the different genera, the 

 oviduct, the spermatheca and ejaculatory ducts. Amongst 

 the social genera, where a worker form exists, its ovaries 

 are less developed than in the ? . The ? is supposed to 

 be able to voluntarily control the passage of spermatozoa 

 into the eggs, so that (J or ? eggs can be laid at will, 

 those unimpregnated always producing S offspring. 



Before closing these short remarks on internal anatomy, 

 I must mention two other important sets of organs in the 

 ? and 5, viz. the poison and wax-secreting organs. The 

 former of these consist of a sac into which the secreting 

 tubes enter, and which can empty its contents through the 



