344 



FARMERS' REGISTER— ON THE GENERATION OF INSECTS. 



more than the excrement of the insect (Aphis 

 humuli) whose propagation we are discussing. 

 "Tlie honey-dew," says ijoudon, "mostly" (we 

 beheve always) "occurs alter the crops have been 

 attackedby these insects."* Sir J. E. Smith, who 

 admits this to be the common cause of honey-dew, 

 contends that what is Ibund on the leaves of the 

 beech is an exception; but he adduces no evidence 

 at ah satisfactory in proof of its being caused by 

 unfavorable winds;t while the undoubted fact of its 

 being the excrement of aphides in so many other 

 instancesj weighs strongly against him. 



A novel theory of honey-dew has just been 

 published by Mr. John Murray, who ascribes it to 

 an electric change in the air. "Last summer," he 

 says, "we investigated the phenomenon whh great 

 care: the weather had been [larched and sultry for 

 some weeks previous, and the honey-dew prevad- 

 ed to such an extent, that the leaves of the cur- 

 rant, raspberry, &c., in the gardens, literally distil- 

 led irom their tips a clear limped honey-dew, ex- 

 creted from the plant; for the phenomenon was 

 observable on those plants that were entirely free 

 from aphides, and so copious was it, Avhere these 

 insects were found, that had their numbers been 

 centuple they could not certainly have been the 

 source of the supply. The question with me, 

 however, was set at rest by applymg a lens, hav- 

 ing previously washed and dried the leaf by a 

 sponge, for m this case the immediately excreted 

 globules became apparent, "§ 



in all observations upon insects, and the other 

 minute parts of creation, it is often exceedingly dit- 

 ficult to distinguish between a cause and an effect. 

 The question of the forniation of honey-dew ap- 

 pears to us particularly liable to erroneous conclu- 

 sions; and we therefore venture to mention a few 

 circumstances which seem irreconcileable with 

 Mr. Murray's ingenious theory. The hop fly 

 (Aphis humuU,') we think, neither does, nor (for 

 want of apj)ropriate organs) can, feed on the ho- 

 ney-dew; and if it did, this feeding would prove 

 rather beneficial than otherwise to the plant, by 

 clearing it from the leaves whose respiratory func- 

 tions it obstructs. So far from feeding on diseased 

 plants and aphis only selects the j^oungest and most 

 healthy shoots, into the tender juicy narts of which 

 it thrusts its beak (haustellum,) which in some 

 species is much longer than the body, and no 

 more fitted for lapping hone}'-dew than the bill of 

 iEsop's crane was for eating out /of a shallow 

 plate. In tl^ experiment, tried by Mr. Murray, of 

 wiping a leaf, might not the leaf have been pre- 

 viously wounded, perhaps, by the beak of some 

 aphis, and hence the exudation of sap, not honey- 

 dew? And may not the circumstances of his find- 

 ing the honey-dew on leaves where there were no 

 aphides be accounted for on the principle that the 

 aphides had abandoned, as they always do, the 

 parts covered with their ejecta, unless these fell 

 from insects on some over-hanging branch? It is 

 justly remarked by M. Sauvages, that they are 

 careful to eject the honey-dew to a distance from 



*Encycl. of Agriculture, p. 865, s. 5444. 



■flntroduction to Botany, p. 189. 



JSee Linn. vol. vi. and Willdenow, princ.of Botany 

 p. 34.3. 



§Treaties on Atmospherical Electricity, p. 147, 

 Lend. 1830. 



where they may be feeding.* We have now in 

 our study a plant of the Chinese chryBanthemum 

 (Anthetuis j/rtcmisi en folia, Willd.,) the young 

 shoots of which have swarmed with aphides all 

 the winter, and the leaves below are covered with 

 honey-dew. We tried the experiment of wiping 

 it off' from a leaf, but no more Avas formed when 

 it was protected by a piece of writing-paper 

 from the aphides above; Avhile the writing-paper 

 became s|)rinkled all over with it in a lew hours. 

 By means of a lens, also, we have actually seen 

 the a[)hi(les ejecting tlie honey-dew. t 



Tile almost instantaneous appearance of these 

 destructive insects in great numbers at the same 

 time, is taken notice of with wonder by almost ev- 

 ery writer. This circumstance, it must be con- 

 fessed, gives considerable plausibility to the notion 

 of their being brought by winds, — for whence, we 

 maybe asked, could they otherwise come? Sim- 

 ply, we reply, from the eggs deposited the prece- 

 ding autumn, which, having all been laid at the 

 same time, and exposed to the same degrees of 

 temperature, are of course all simultaneously 

 hatched. In the case of the aphides, also, the 

 fecundity is almost incalculable. Reaumur provett 

 by experiment, that one aphis may be the progen- 

 itor of 5,904,900,000 descendants during its life; 

 and Latreille sa}^, a female during the summer 

 months usually produces about twenty-five a, day.- 

 Reaumur further supposes, that in one year there 

 may be twenty generations. We ourselves have 

 counted more than a thousand aphides on a single 

 leaf of the hop; and in seasons when they are 

 abundant — when every hop-leaf is peopled with 

 a similar swarm — the number of eggs laid in au- 

 tumn must be, to use the words of" Good, "my- 

 riads of myriads." The preservation and hatching 

 of these eggs in the ensuing spring must, it is ob- 

 vious, depend on the weather and other accidental 

 circumstances, seldom appreciable by our most 

 minute observations.^ 



The history of other insects, erroneously referred 

 to blighting winds, is more easily traced, from 

 their being of a larger size than the aphides. The 

 caterpillar, for example, of Lozotania Rosana,- 

 mentioned before, which rolls the leaf of the rose- 

 tree, is one of this kind. It is well known as fur- 

 nishing the common poetical comparison of "a- 

 worm i' the bud." Early in autumn the mother 

 insects deposite an irregularly oval-patch of yel- 

 lowish eggs, covered with a cement of the same 

 color, sometimes upon the branches of the rose- 

 tree, but more frequently, as we have observed, 

 upon some smootli object contiguous. For seve- 

 ral succcessive seasons, we have found more than 

 one group of these eggs upon the glass panes, aa- 

 well as the frame-work, of a window, beneath- 

 which a rose-tree has been trained. At present 

 (January 1830) there are tw^o of these groups on 

 one pane, and three on the frame-Avork; and as 

 each contains about fifty eggs, should they all be 

 successfully hatched, tAvo or three hundred cater- 

 pillars Avould at once be let loose, and, steraming 

 down simultaneously upon the rose-tree beneath, 

 Avould soon devour the greater number of its buds. 

 As this AvindoAv faces the east, the sudden ap- 

 pearance of the insects Avould make it appear not 

 unplausible that they had been SAvcpt hither by an 

 easterly Avind. 



fTrans. Soc.Roy. de Montpellier. 

 *J. R. X5. R. 



