18 INTRODUCTION. 



rangement ; the reproductive glands are symmetrical 

 and double, the efferent ducts join a common duct 

 before opening. The female sexual organs consist 

 generally of the ovaries, oviduct, uterus, and vagina ; but 

 there is often a large number of accessory appendages, 

 sometimes present, sometimes absent. Indeed it is seldom 

 that all parts are present together, one or several being 

 wanting. In neuter bees (barren or undeveloped females), 

 the ovaries are deficient, though the evacuating ducts are 

 constant. The females are usually larger than the 

 males ; this is strikingly the case where the females are 

 wingless, the males winged ; the antennae and the tarsi 

 often differ considerably in the sexes. Insects are 

 generally oviparous, though some are ovo-viviparous. 

 Various forms of agamogenesis, that is to say, pro- 

 duction without the union of the sexes, have been 

 observed amongst insects. Females with a reproductive 

 apparatus provided with a receptaculum seminis may 

 produce either embryos, as Lecanium hesperidum and 

 Chermes ahietis amongst the Coccina, or ova as Psyche 

 helix, Solenobia lichenella^ and >S'. triquetrella amongst 

 the Lepidoptera ; or they may produce wingless queen 

 bees and winged queens as amongst Hymenoptera. 

 In this class of cases sexual may alternate with asexual 

 production, and it is most curious to observe that all 

 male bees are produced from unfertilized eggs, while 

 only the fertilized bee-egg will develop into a female or 

 a perfect queen. Again, females with reproductive ap- 

 paratus more or less imperfect, may produce either eggs, 

 as happens with the "workers" or neuters amongst bees, 

 whose produce is probably always males, or they may 

 produce embryos as is the case with the Aphides, in 



