575 



The egg-tubes differ very much in number and length. In all the 

 larger orders, except perhaps the Lepidoptera and Heteroptera, some 

 species have very few, while others possess a great many. Thus in 

 Coleoptera, Lytta vesicatoria has a great many, Lixus has only two ; 

 in Orthoptera, Acheta domestica has a great many, while in a small 

 Locusta I only found six ; in Neuroptera, Libellula has a great many, 

 Psocus only five ; of the Diptera, the majority have many, Melo- 

 phagus only two ; in Homoptera, Coccus has a great many, while 

 Aphis padi has only three ; in Hymenoptera, Apis mellifica has about 

 1 70, and Chelonus has only two ; and even in so small a group as the 

 Dermaptera, Labidura gigantea has, according to Leon Dufour, only 

 five, while Forficula auricularia has a great number. 



The number of egg-germs in each egg- tube differs also very much. 

 The Lepidoptera, in which the number of egg-tubes is very small, 

 have a great number of egg-germs in each, while the Homoptera, in 

 which the egg-tubes are so numerous, have very few egg-germs in 

 each tube. On the other hand, in Heteroptera, the number varies 

 very little ; while in Coleoptera, Hymenoptera, and Diptera, it differs 

 greatly, though not so much as that of the egg-tubes. The number 

 of egg-germs, is, however, by no means so easy to determine as that 

 of the egg-tubes. It is probable that in each species the number 

 is definite, except perhaps where it is very numerous, as, for instance, 

 in certain Lepidoptera which have more than a hundred. In most 

 egg-tubes, however, the egg-germs become so " small by degrees," 

 that it is almost impossible to say exactly how many there are. 



Each egg-tube consists generally, if not always, of two membranes. 

 The outer or muscular one is very evident in Hymenoptera, Geode- 

 phaga, Diptera, and indeed in most insects, but in some cases I could 

 not distinguish it. The inner membrane is delicate and structureless. 

 On its inner side lies a layer of epithelial cells, which in most parts 

 form a continuous layer ; but in those insects which have a group of 

 vitelligenous cells between each of the egg-germs, they are at these 

 parts more sparingly distributed. 



These epithelial cells probably take an active part in the secretion 

 of the yolk in all insects, and are the principal, if not the only organs, 

 which form the yolk in Orthoptera, Pulex, and t^ie Libellulina. 



Between each of the egg-germs in Lepidoptera, Diptera, Hyme- 

 noptera, Geodephaga, Hydradephaga, and Neuroptera (except the 



