October 2. 1893.] 



KNOWLEDGE. 



187 



species called Sdiiioneura ulmi, which makes galls on the 

 leaves of elm trees. The eggs of this creature are ex- 

 ceedingly minute yellowish or brownisli bodies, which are 

 to be found, if diligently sought for at the approach of 

 winter, in the crevices of the barks of trees which have 

 borne the insect during the preceding summer. Each of 

 these eggs represents the entire family of a single pair of 

 insects. The mother was but little larger than, the egg 

 she produced, and she died immediately upon its laying. 

 Sometimes, indeed, she dies before her solitary egg is laid, 

 in which case the egg would be found in such situations as 

 above described, but enclosed within the dried skin of its 

 parent. Of course one would naturally expect that when 

 these eggs hatch they would give rise to insects similar to 

 those which produced them, or at least to creatures which 

 by a process of development and growth would gradually 

 become like them. But when we are dealing witli 

 aphides, such ideas must be put aside altogether, for here 

 we fiud developed to a wonderful extent the phenomenon 

 called " alternation of generations," a phrase by which is 

 meant that forms, more or less different from one another, 

 regularly succeed one another in a cycle by some process 

 of multiplication, so that a number of " generations," so to 

 speak, intervene before we arrive again at the same sort of 

 thing that we started with. In the present instance, no 

 less than seven such generations constitute the cycle, and 

 these must successively appear, each being the viviparous 

 parent of the next in the scale, before we arrive again at 

 the egg with which we commenced. Some of these seven 

 are pretty much alike, but the rest differ greatly. 



It is obvious that if we are to speak intelligently of these 

 difi'erent stages in the life-history of a single insect, we 

 shall require distinctive names for the entire seven. No 

 one of them, to the exclusion of the others, can be de- 

 scribed as the insect itself; nothing short of the whole 

 series can fairly be considered as representing this. Nor 

 are they stages like the larva and pupa of other insects, 

 the differences between which are no more than what are 

 caused by a mere change of skin, so that the insect remains 

 one indivisible creature throughout its life. On the con- 

 trary, in each one of these stages the insect may be 

 represented by an indefinite number of separate and 

 independent beings, which in fairness ought all to be 

 taken into account in estimating the produce of a single 

 egg, up to the time when the next egg appears. Of course 

 these stages might be distinguished numerically, but 

 names descriptive of their functions are preferable. Such 

 a system of names was devised by Lichtenstein. They 

 were expressed in French, and no thoroughly satisfactory 

 English equivalents seem to be available. Not to worry 

 our readers, however, with these rather complicated 

 foreign expressions, we will render into English the most 

 characteristic portion of the names, and thus we speak of 

 the creature which issues from the above-mentioned egg 

 as the " scem-mother " or "foundress." Buckton, the 

 historian of the British aphides, gives this stage the name 

 of " queen aphis," in consequence of certain resemblances 

 between her functions and those of queen bees or wasps. 

 If this name be used, however, it must be borne in mind 

 that while queen bees and wasps are fully developed 

 females which mate with male partners and produce eggs 

 in the usual way, nothing of this sort takes place in 

 connection with the queen aphis. On the whole, it will 

 probably be productive of less confusion to use the term 

 " stem-mother," implying by this that the form so de- 

 signated is the first active six-legged member of the series, 

 and that she is, as it were, the mother of the stock, the 

 progenitor from whom the whole pedigree is derived. 



The egg ^hatches in springtime, and the stem-mother, 



who has no wings and is thei'efore not likely to wander 

 far, at once begins to attack the young leaves on the tree 

 on which she was hatched, piercing them with the fine 

 needles in her beak as soon as they unfold from the bud. 

 The damage done to the tissues by the punctures induces 

 an abnormal growth, and little cavities appear on the leaf, 

 within one of which the stem-motlier nestles. The leaves 

 that have been attacked in this way now curl downwards 

 and change colour, becoming pale and sickly-looking. 

 Meanwhile the stem-mother continues to suck out their 

 juices, grows, changes her skin two or three times, and 

 after a few days begins to produce a family, with which 

 she stocks the cavities of the curled leaves. They are 

 little green things and are produced alive, so that no time 

 is wasted in getting them set to work at the task of tapping 

 the leaves of their juices. Some of these members of the 

 second generation remain on the leaf which was their 

 birthplace, but others crawl oft' to neighbouring leaves ; 

 ultimately they produce viviparously another set of off- 

 spring, at first similar to themselves, but destined to 

 advance to a higher grade of development. By successive 

 moultings they first acquire the rudiments of wings, and 

 at length become fully equipped with four delicate trans- 

 parent wings. These winged forms are all of the same 

 sort, and all have the power, like their predecessors, of 

 producing li%'ing young. They are known as " emigrants," 

 because they soon make use of the power of flight which 

 they possess, and float away to other ti'ees, thus spreading 

 their ravages over a wider area. By this time, then, we 

 have a large number of creatures as the descendants of a 

 single egg, spread far and wide, and yet no other egg has 

 thus far appeared, nor indeed will do so till the end of the 

 summer. 



These winged emigrants do not live long, but each gives 

 birth to about a dozen living young, at intervals of about 

 half-an-hour, and then dies. We are now in the fourth 

 generation, and these new creatures are very difi'erent from 

 those that have preceded them, being wingless and very 

 active. When first dropped they look much like eggs, 

 and have to shed a thin skin before they can move their 

 limbs ; the casting of this membrane enables a number of 

 minute hairs to shoot rapidly out from the surface of the 

 body, and thus we have a kind of transformation scene 

 going on, a little shining yellow stationary object changing 

 in about half a minute to a hairy six-legged creature, 

 which runs about quickly and soon covers itself with a 

 kind of cottony down. Another generation of wingless 

 creatures is again produced from these, and then we come 

 in the sixth generation to something like the emigrants 

 again, i.e. the members of this grade gradually acquire 

 wings ; but their function is quite difl'erent fi'om that of 

 the emigrants aforesaid, for they are to become the parents 

 of the true males and females which, in the next genera- 

 tion, will appear for the first time. These winged creatures 

 are smaller tlian the previous set, and are called by the 

 rather inappropriate name of "pupa-bearers." They get 

 their wings in -July, and quickly make good use of them to 

 wander in search of suitable places for the deposition of 

 their brood. They settle on the bark of the selected elm 

 trees, and there give birth to two sorts of beings, both 

 exceedingly minute, and yellowish or orange in colour. 

 These, which constitute the seventh generation, are the 

 true sexes. Their sole function is that of reproduction, 

 and hence all their energies are bent to this purpose. 

 They take no food, and in fact the female has neither beak 

 nor mouth, and her partner is almost in a similar predica- 

 ment, for though he is born with a beak, he soon loses it 

 and becomes as mouthless as his mate. 



After having paired, the female establishes herself in a 



