AGAMOGENESIS IN INSECTS. 383 



from the first, be a fold of the blastoderm and thus consist of 

 two layers, the inner of which is continuous with the sternal 

 band, and the outer with the blastoderm which invests the 

 tergal surface of the vitellus. In the latter case, it becomes 

 strictly comjuarable to the amnion of a vertebrated animal ; 

 and, when the folds have united in the middle line, the invest- 

 ment in question is distinguishable into an outer membrane, 

 which answers to the lamina serosa, and an inner, which cor- 

 responds with the amnion proper of the vertebrate embryo. 

 In some cases, the vitelline substance fills up the interval be- 

 tween the lamina serosa and the amnion, so that the sternal 

 band and the latter form a sac plunged into the interior of 

 .the yelk. 



The development of a more or less complete amniotic in- 

 vestment has been observed in Orthoptera {Libellida), Cole- 

 optera, Hemiptera, Ilymenoptera, Lepidoptera, and Dip- 

 tera, but it does not appear to be universal. 



Agamogenesis is of frequent occurrence among insects, 

 and occurs under two extreme forms ; in the one, the parent 

 is a perfect female, while the germs have all the morpho- 

 logical characters of eggs, and to this the term parthenogene- 

 sis ought to be restricted. 1 In the other the parent has in- 

 complete female genitalia, and the germs have not the ordi- 

 nary characters of insect eggs. 



In Coccus (Lecaniuni) hesperidam, in Chermes abietis 

 and pint, no males have been observed; but the perfect 

 females produce ova, out of which only females proceed. It 

 is probable that many species of gall insects ( Cynips) are in 

 the same predicament. 



The unimp.regnated, apterous, caterpillar-like females of 

 the Lepidopterous genera Psyche and Solenobia lay eggs 

 out of which only females issue. The males occur but rarely 

 and locally, and, from the impregnated eggs, males and 

 females issue in about equal numbers. 



Leuckart discovered that the ovaries of so-called neuters 

 among wasps, hornets, humble-bees, and ants, often contain 

 more or less well-developed eggs, and that in the wasps and 

 humble-bees such eggs are laid and develop young, the sex 

 of which was not ascertained. Von Siebold has observed that 

 the neuters of Polistes gallica are distinguished from the per- 



1 The excellent " Beitrage zur Parthenogenesis " (1871) of Von Siehold is 

 my chief authority for the statements in the text respecting Agamogenesis in 

 Insects. 



