148 



HARDWICKE'S SCIENCE-GOSSIP. 



in the aphides, aud that they will again multiply 

 should the plant exhibit fresh signs of sickliness. 

 The suddenness with which they make their appear- 

 ance, and the vast numbers acquired in a very brief 

 period, are wonderful : the plants in a whole field 

 of beans will appear to be covered in a few days ; 

 but this will be less astonishing when we consider 

 the mode of reproduction and the rapidity with 

 which it takes place. Sexual intercourse takes 

 place only in the autumn, after the insects have 

 become fully developed ; the female then deposits 

 her eggs in some secure spot, where they remain 



Fig:. 93. Wingless Aphis (tnagiiifled), 



dormant through the winter, the first genial days 

 in spring bringing forth not the perfect-winged 

 form which produced the egg, but the well-known 

 wingless form which is so familiar to us all. This 

 wonderful fertile larval form is possessed of great 

 fecundity, but it is its mode of reproducing which 

 is so remarkable : this is carried on by internal 

 gemmation, the young one being produced alive, 

 aud differing from its parent only in size. This 

 process of development goes on for several gene- 

 rations without any change in the form of the 

 insect ; at the close of the summer, however, the 

 fertile forms which have not reproduced themselves 

 pass, by metamorphosis, into the perfect insect, 

 males and females, for the most part winged. These, 

 after pairing, produce the eggs which are to be 

 hatched in the ensuing spring, after which they die. 

 During the summer numbers complete their meta- 

 morphosis, and appear as winged insects. The 

 larval form which will undergo the final meta- 

 morphosis may easily be recognized, the bulbs in 

 which the wings are inclosed being visible on either 

 side the thorax. I have separated many of these, 

 and placed them under a glass shade, but in no 

 instance have they reproduced, the development 

 continuing until, after the final shedding of the skin, 

 they have appeared as fully-developed winged 

 insects. These have never produced eggs, and I 

 ihink were all males; it seems probable that 



although winged insects are produced during the 

 summer, it is not till the autumn that the perfect 

 females are produced ; all the fertile larval females 

 which I have kept, either alone or under the same 

 shade with the winged form just mentioned, having 

 increased rapidly in the usual way, and in no 

 instance produced eggs. The perfect insect is very 

 different from the larval form, and under the mag- 

 nifying-glass a very handsome object, with six long 

 legs, graceful antenna, shining wings and spotted 

 sides, also a pair of curious projecting tubes, stand- 



Fig. 93, Winged Aphis (magnified). 



ing out like horns, near the extremity of the abdo- 

 men, of which more hereafter. This intermediate 

 but fertile form of the aphis is an example of the 

 nursing system of Steenstrup, observed also in the 

 vegetable kingdom in the case of the ferns and 

 horse-tails, which increase by spores, which, falling 

 into the ground, produce the prothallium. The 

 prothallium never becomes a fern, but on it are 

 formed the true generative organs which produce 

 the embryo afterwards developed into the perfect 

 plant. This case differs from the aphis in the 

 nurse-form (the prothallium) producing the gene- 

 rative organs. 



The echinoderms, &c., exemplify other varia- 

 tions of the same phenomena. Mungo Ponton 

 thus describes the fertile nurse-form and the way 

 in which it is illustrated by the aphis: — "By fertile 

 nurse-form is to be understood an organism which 

 becomes fertile without ever attaining, or at least 

 before having attained, the perfect form due to its 

 species. ... In simple metamorphosis, it is one 

 and the same individual organism that passes 

 through all the stages, from the form which it 

 wears on leaving the egg, to that which it assumes 

 on attaining the perfection belonging to its species ; 

 but in the case of a fertile nurse-form, it is in general 

 onlythose forms which are produced bythe nurse that 

 ever attain the specific type. So likewise, in simple 



