382 ANIMAL LIFE-HISTORIES 



generations of females, some wingless, others possessing wings, 

 which bring forth their young alive, i.e. are viviparous. These 

 are all produced from unfertilized eggs. But on the approach 

 of winter egg-laying or oviparous females come into existence, 

 and also a certain proportion of males: in either sex wings may 

 be present or absent. Fertilized eggs are now laid, which hatch 

 out into viviparous females the following spring. We have 

 here another example of the phenomenon of " alternation of 

 generations ", already illustrated by some of the Ccelenterates 

 (see p. 349). But there is a great difference between the two 

 cases. For in Ccelenterates vegetative propagation and egg- 

 production alternate in the life-cycle (metagenesis], while in 



Aphides there is alternation 

 between two kinds of egg, 

 i.e. unfertilized eggs which 

 develop internally, and fer- 

 tilized eggs which develop 

 externally. A life-cycle of 

 this kind is technically de- 

 scribed as a case of heter- 



Fig. 90j.-Turnip- Flower Aphis ^pkis floris-rapa\ 1,2, Ogeny. If WC denote the 



Winged female; 3, 4, wingless female. 2 and 4 indicate the sta p- e producinP" Unfertilized 

 natural size. O JT o 



eggs by Ei, and that pro- 

 ducing fertilized eggs by 2, the formula Ei Ei Ei Ei, &c. &c., 

 2 will represent the life-history, a large number of viviparous 

 generations (Ei) succeeding one another before the single ovi- 

 parous generation (2) is reached. 



Some of the aquatic bugs carry about their eggs with them 

 until the time of hatching. This has been observed for one 

 of the very few marine forms (Halobates\ and has also been 

 described for certain freshwater species (e.g. Diplonychus). In 

 the latter case the eggs are sometimes, if not always, cemented 

 to the back of the male. 



Particular interest attaches to the eggs laid by the members 

 of one family of freshwater Bugs (Nepidcz, fig. 903). In a 

 Water-Scorpion (Nepa] the egg is provided with a tuft of threads 

 at one end, and is deposited in the stem of a water-plant, the 

 threads alone projecting. It has been suggested that they are 

 of use in conducting air to the developing embryo, for they 

 are of spongy nature, and continuous with a layer of the same 



