42 IlITINO AND NOXIOUS INSECTS OTHKU THAN MOSQUITOES 



propirly, it is not iiii t-nsy iimttcr to iinswi-r nil Major Dickinson's queries. One ciin, liowiver, 

 iisstTt tliiit forms are found on tlie lioncy-covcred surfaces. I liuvc discovered eggs in tliis 

 situation as well as winged forms. liutli these are e.xtnniily niiiiutc and might easily escape 

 naked-eye observation. The forms found on the under-surfaee of the leaf are usually apterous 

 females, and the time of year at which they are found will proliaMy indicate wlictlitr they 

 are parthenogeiietie females or engaged in oviparous or vivi|)an)us repniductiiJii. The 

 life-history of Aphidi'S is so very coiuiilix and vit of such amazing iiitinst, and the 

 .1. xonilii is such an important factor in cereal cultivation in tlir Sudan that I here take the 

 liberty of inserting sonic extracts from Insects, Part II., Camliridge Natural Historv. hv 

 Dr. David Sharj). 



'■ Thr inijiviihia! life fur sivcral gcnrratimis is rrstrictid to constant, or at anv rate 

 copious, imhiliition of food, accomi)anicd liy an almost uninterrupted produetioii of young by 

 parthenogenetic females, the young so produced Incoming rapidly (sometimes in the course 

 of eight or ten days, but nn)re usually in about twenty days*) themselves devoted to a 

 similar process; so tiiat in the comparatively slmrt ]m ridd of a few months the progeny 

 resulting from a single individual is almost iniiunicralilc. This nniarkable state of afl'airs is 

 accompanied by other peculiarities of pliysiology, with the result that the life-histories of 

 successive generations become very diverse, and <'omple.\ cvi les of series of generations 

 diflering more or less from one another are passed tliruugh, tiie species finally returning to 

 bi-sexual reproduction, and thus inaugurating aiintlier cvile of generations. Tlie surprising 

 nature of these facts has in the last InO years caused an immense amount of discussion, but 

 no sati.sfactory light has yet been thrown on 1i<o conditions that really give rise to the 

 exceptional phenomena. These phenomena are: (I) parthejiogenesis ; (2) oviparous and 

 viviparous reproduction; (3) the production of generations (if individuals in whicii tho sexes 

 are very uiie.|Ually represented, males being frequently entirely absent ; (4) the proiluction of 

 imlividuals ditVering as to the aciiuirement of wings, some remaining entirely apterous, while 

 others go on to the winged form ; (5) the production of individuals of the same sex with 

 different sexual organs, and liistinctions in the very early (init iHit the earliest) stage of the 

 formation of the individual; ((i) iliflferences in the life-habits of successive generations; 

 (7) differences in the habits of individuals of one generation, giving rise to the phenomenon 

 of parallel series. All these phenomena may occur in the case of a single species, though in 

 a very variable extent. 



The simple form of Aphid life may be described as follows: — 

 Life History of Eggs are laid in the autumn, and hatch in the spring, giving rise to fenialis of an 



Apliida! imperfect character having no wings ; these produce living young parthenogeneticallv, 



anil this process may be repeated for a few or for many generations, and there mav be 

 in these generations a greater or less number of winged individuals, and perhaps a few males. 

 (There is some doubt on this point, as the I'ariier ob.servers seem to have snppo.sed that a 

 winged individual appearing in a generation chiefly apterous was ipso fucto, a male; it seems, 

 however, to be certain that perfect wingeil nuiles appear in some species in generations 

 producing no perfect sexual females. Speaking geiu'rally, the coiirse of events seems to be 

 that in summer there exist only wingless and winged parthenogenetic females, and that the 

 sexually perfect forms appear for the first time in autunni.) After a time when temperature 



•This npplicx to what occurs in a tompemto climate. Under tropical conditions production is prol>nl>ly 

 Diuch more rapid {ridr p. 40) Mr. Loimxliurj-'s remarks. — A.B. 



