REPRODUCTION 181 



viously a sexual one. From the fact that only one parent is concerned 

 it is said to be unisexual reproduction, more commonly known as partheno- 

 genesis. There are many animals which employ parthenogenesis. 

 Among these may be mentioned various species of rotifers, many small 

 Crustacea, and insects of certain orders. Some insects which- employ 

 parthenogenesis are the well known plant lice or aphids, and many species 

 of Hymenoptera, as ants, bees, and wasps. The method has also been 

 observed in a few moths, a few of the Coccidae, and commonly among the 

 Thysanoptera (of which Thrips is an example). The hfe cycles of some 

 of these species are remarkable. 



The females of many parthenogenetic species produce, for a number 

 of generations, only females. At intervals, frequently in the fall, males 

 are also produced which fertilize the eggs. These zygotes usually differ 

 from the unfertilized eggs in being provided with hard shells and in being 

 resistant to the rigors of a winter season. The fertilized eggs hatch in 

 the spring into parthenogenetic females which repeat the cycle as out- 

 lined. Many species of aphids and of the lower Crustacea have cycles 

 of this type. In certain species the bisexual reproductive phase is 

 apparently entirely omitted and reproduction is exclusively partheno- 

 genetic. Thus the black flower thrips, Arithothrips niger, the brown 

 chrysanthemum aphid, Macrosiphum sanhorni, many species of Coccidae 

 or scale insects and some Cynipidse or gall-producing insects never 

 produce males. In some species of Hymenoptera, as the ants, bees 

 and wasps, both males and females are usually produced. The female 

 lays both fertilized and unfertilized eggs, in some way controlling fertili- 

 zation of the eggs by the release or retention of spermatozoa stored in 

 the seminal receptacles. Among bees the males (drones) are derived 

 from unfertihzed eggs, the females (queens and workers) from fertilized 

 eggs. 



Paedogenesis. — Sexual reproduction is usually carried on only by 

 the adults, though this is not always the case, for there are certain species 

 whose members have the remarkable power of reproducing sexually while 

 they are in the larval state. This reproduction by an immature animal 

 is called pcedogenesis. Paedogenesis may be of two types, parthenoge- 

 netic or bisexual. 



Miastor and csrtain species of Cecidomyia, flies belonging to the 

 family Cecidomyiidae, may be taken as examples of parthenogenetic 

 paedogenesis. The larvae (Fig, 139) produce ova which develop by 

 parthenogenesis into larvae before the oviducts are present. The latter 

 generation of larvae escape from the parent by rupture of the body wall. 

 Several generations may be produced in this fashion; then the larvae 

 pupate and emerge as normal adult males and females. 



Many of the trematodes, parasitic flatworms living in various organs 

 of vertebrate animals, have stages considered by many to be larval, which 



