EFFECTS ON THE HIGHER INVERTEBRATES 29 



factors, but that the production of males during inanition is probably only an 

 indirect effect. 



The relation of nutrition to sex has also been studied extensively in the 

 crustacean water-fleas by Leydig and others. The change from parthenogenetic 

 to sexual mode of reproduction under unfavorable conditions (especially inani- 

 tion) in this group was noted by Kerherve ('92), Cuenot ('94) and Issakowitsch 

 ('05). More recently, the internal factors have been emphasized, although it is 

 admitted that the hereditary tendency may be influenced by external factors, 

 such as malnutrition, especially during labile period of the ovum (McClendon 

 '10; Woltereck 'n). Green ('19) recently concluded that in Simocephalus 

 the sex is probably predetermined in the ovary; but is also subject to environ- 

 mental influence, though probably not through starvation. 



Among insects the relations of nutrition to sex have been noted especially 

 in the aphids (plant-lice), the Lepidoptera and the Hymenoptera. The appear- 

 ance of winged male forms in aphids as a result of underfeeding was noted by 

 Kyber (1813), Leydig, and others. The same phenomenon in the grape-louse 

 Phylloxera was found by Keller ('87) and Behr ('92). Thus apparently all 

 observers admit the effect of unfavorable environment, especially underfeeding, 

 in causing the cessation of parthenogenesis and the appearance of males and 

 sexual reproduction among the aphids. The conditions in this group therefore 

 form strong evidence supporting the theory of nutritional sex-determination. 



In the Lepidoptera, Treat ('73) observed a preponderance of males as a 

 result of underfeeding the larvae of certain butterflies. This was not confirmed 

 by Poulton ('93), although he admitted that a lesser resistance to inanition in 

 the female larvae might result in a selective mortality with survival of a relatively 

 larger number of males. This, of course, is not strictly a process of sex-deter- 

 mination, but rather of sex-survival. In the underfeeding experiments by 

 Kellogg and Bell ('03, '04) on the silkworm Bombyx, and by Holmes ('10), 

 Guyenot C13) and Loeb and Northrop ('16) on the fruitfly Drosophila, no 

 effect on the sex-ratio was observed. 



Among the termites and Hymenoptera, the sex is known to be determined by 

 fertilization (the unfertilized eggs producing the males). The diet, however, 

 determines the development of the female reproductive tract, which in the few 

 larvae well-fed (with "royal diet") becomes functionally developed, producing 

 the queens;" while in the less richly nourished larvae the reproductive tract 

 remains rudimentary, producing the "workers," of which there may be different 

 varieties, according to the amount of food available. There is some evidence 

 indicating, that in wasps (and possibly other Hymenoptera) the sex-ratio may be 

 affected by nutrition, but this is somewhat uncertain. 



Among the invertebrates in general, we may therefore conclude from the available 

 evidence that malnutrition tends to favor the sexual, rather than the asexual, mode 

 of reproduction. Furthermore, especially in Hydra, rotifers, daphnids and 

 aphids, the sex-ratio is at least to some extent subject to environmental influence, 

 inanition tending to produce a preponderance of males. Wilson ('00) concluded 

 that nutrition is one factor which may determine sex. Schultze ('03) reached 

 this conclusion for Hydra and Hydatina, but felt less certain regarding the 



