46 



AN ECONOMIC ENTOMOLOGY. 



Fig 



are predaceous, and live upon others of their kind ; great num- 

 bers are parasites, their prey being other insects of all orders, 

 and in this way forms that depend upon plant life for their sub- 

 sistence are kept within definite bounds. These bounds, though 

 they may vary from year to year, never change much except 

 where man interferes. This subject will be touched upon in an- 

 other place more fully. 



Except in rare instances insects are of two sexes, male and 

 female, and we nowhere find among them true hermaphrodites. 

 In the plant-lice and gall-wasps we have apparent exceptions ; 

 but these are only apparent. 



In the female the most prominent sexual organs are the ovaries. 

 These consist of two sets of tubes, one in each side of the abdo- 

 men, usually below the digestive system, united 

 at their base into one larger tube called the 

 oviduct ; the two oviducts unite, just before 

 opening outwardly, into a single chamber 

 called the vagina. The vagina opens out- 

 wardly at the end of the body, sometimes with- 

 out special modification, sometimes by means 

 of a flexible or extensile tube, sometimes as a 

 long, rigid cylinder, and occasionally in the 

 form of a sting. These structures, be they 

 rigid or otherwise, are always called ovipositors, 

 and their function is to place the egg into the 

 position necessary for its best development. 

 Associated with this system are a number of 

 glands, the use of which is either to give a 

 sticky coating to the egg, enabling it to adhere 

 to the leaf or other point at which it is laid, or 

 to supply a poisonous secretion, where such is 

 necessary for defence or for stupefying prey. 

 There is also a little sac, attached by a slender 

 duct to the vagina, at about the point where 

 the two oviducts unite. This is the seminal 

 receptacle, and in this is received the seminal 

 from it the eggs are fertilized as they pass from 

 the oviduct into the vagina on their way through the ovipositor. 

 In some insects these receptacles are very large, especially where 



Ovarian tubes of one 

 side, in Polistes, show- 

 ing eggs in all stages 

 of development, with 

 nutritive cells, n m, be- 

 tween ; r s, seminal re- 

 ceptacle ; ov, oviduct ; 

 vag, vagina. 



fluid of the male 



