cxii Introduction. 



The segments of the body posterior to the eighth abdominal may- 

 be modified so as to serve in either sex as ' ovipositors' or as intro- 

 mittent organs. There are no hermaphrodite insects, and sexual 

 reproduction is the rule in the class. 



Several forms of agamogenesis have been observed amongst 

 Insects. In one of these, females with a reproductive apparatus 

 provided with a receptaculum seminis produce (without any congress 

 with males), either embryos, as Lecanmm hesperidum, Ckermes abietis, 

 amongst the Coccina ; or ova, as, amongst Lepidojitera, Psyche helix, 

 Solenohia Uchenella, and Solenohia triquetreUa, and as, amongst 

 Hymenoptera, Cpiips, apterous Queen bees, and normal winged 

 Queen bees before they leave the hive. Jn this class of cases sexual 

 may alternate with asexual genesis, and it is to be noted that the 

 male offspring of the Queen bees are only and exclusively due to 

 the agamogenetic process. In a second class of cases, females with 

 a more or less imperfect reproductive apparatus produce either ova, 

 as is the case with the ' workers' amongst the social Hymenoptera, 

 Apis mellifica, Vesjia, Bonibns, in which the vagina as well as the 

 receptaculum seminis is I'udimentary, and which, with, possibly, the 

 exception of the Vespidae, always produce males; or embryos, as 

 is the case with Aphis, in which certain generations without sper- 

 mathecae or coUeterial glands are viviparous agamogenetically, 

 whilst others with a perfect sexual apparatus are oviparous gamo- 

 genetically. This form of asexual genesis is called ' pseudopartheno- 

 genesis,' and the reproductive gland a ' pseudovarium.' Asexual 

 genesis was supposed to take place metagenetically, that is to say, 

 by a process of internal gemmation in a non-differentiated part of 

 the body, in the larvae of certain Diptera of the family of Ceci- 

 domyidae; but the discovery, by Leuckart, of specialized germ- 

 producing organs in these animals appears to show that the only 

 differences between this process and that observable in Aphis consist 

 in its taking place in forms of a holometabolous order, which, as 

 being larvae, are very different in external appearance from the 

 perfect sexual insect, and in the ' pseudovaria ' being destitute of 

 any ' pseudoviduct.'' 



In the development of the ovum the yolk is surrounded by a 

 germinal membrane, in which the first traces of the head and the 

 ventral half of the embryo make their appearance as the so-called 

 * primitive streak.' Insects are ordinarily oviparous, but they may 



