REPRODUCTION AND HEREDITY 



143 



The number of ovarian tubes and the rate at which the 

 eggs ripen varies enormously in different insects. Some 

 female beetles have only two tubes to each ovary ; most 

 moths have four ; other beetles like the chafers have six ; 

 cockroaches eight ; while a queen-bee has about a hundred 

 and fifty and a queen-termite over two thousand tubes on 

 each side of the body. The ovarian tubes of either side 

 open into an oviduct and the two oviducts lead, in most 

 insects, into a median external passage, the vagina, which 

 has a lining of cuticle being formed by an inpushing of the 

 outer skin. The reproductive passages of insects always 

 open towards the hinder end of the body, and the vaginal 

 aperture is usually situated on or immediately in front 

 of the eighth abdominal segment. Just behind it is the 

 opening of the spermatheca or reservoir into which the 

 sperm-cells pass when the female pairs with a male insect. 

 This spermatheca may be a simple ovoid or sub-globular 

 chamber provided with a short duct, or consist of two or 

 three chambers (Fig. 34, sp) with relatively long ducts ; 

 the whole apparatus is lined with cuticle. The typical 

 insect ovipositor consists of three pairs of processes, one of 

 which belongs to the eighth and two to the ninth abdominal 

 segment ; these acting as a forceps hold the egg that is 

 being laid (Fig. 35). The processes of the ovipositor may 

 be relatively short, and usually unseen because retracted 

 into a pouch formed by the inpushing of the hinder 

 abdominal region, or they may project conspicuously at the 

 tail-end of the insect as in ichneumon-flies and many grass- 

 hoppers (Fig. 36). In most two- winged flies (Diptera), the 

 processes of the ovipositor are very short, but owing to a 

 great development of the intersegmental cuticle in the 

 hinder part of the abdomen, that region can be extended 

 in a ** telescopic " manner when eggs are being laid and 

 retracted again when the organ is out of use (Fige 35). The 

 well-known sting of wasps, bees, and their allies is a highly 

 specialised ovipositor modified into a formidable weapon of 

 defence or attack, but in some cases retaining still its 

 original function as an egg-laying organ. 



