^B6 Retrospective Criticism, 



time of its fabrication ? If such were not the case, it is necessary, in 

 order to ascertain the true mode employed in the formation, to bear in 

 mind the habitation of the larva ; upon which point Latreille, in his Histoire 

 Katurellcy &c., vol. ii. p. 324., states that the species of Donacia are generally 

 found " sur I'iris et le roseau j ce qui fait croire que leurs larves, qui ne 

 sont pas connues, vivent dans la tige de ces plantes. Selon Linnaeus, la 

 nymphe de la Donacie crassipede, qui a la forme d'une coque brune, est fixee 

 sur la racine de la Phellandrie." * And Kirby and Spence (vol. iv. p. 500.) 

 more decidedly state that " on aquatic plants you must look for Helodes, 

 and the splendid Donaciae, which, living on submerged shoots and roots 

 of these plants in their larva state, continue to attend them when perfect." 



The question is one of considerable interest, involving as it does the 

 method in which other aquatic insects, such as the Dyticidae, Hydrophilidae, 

 &c., construct their cocoons, as well also as the method in which the cases 

 of the grubs of the caddice flies (Trichoptera) are formed. I have not 

 however at present an opportunity of referring to the observations of De 

 Geer, Reaumur, &c., upon these points. I remain. Sir, &c. — John O. 

 Westwood. Chelsea, March 22. 1830. 



P. S. I find that the Cynips quercus-folii occasionally remains in the 

 larva or pupa state through the winter, my friend Mr. Ingpen having dis- 

 covered some of the galls upon leaves lying upon the ground, so late as 

 the 28th ult., each of which, when opened, contained a single pupa. — 

 J.O.W, Aprils. 1830. 



The Ascent of aeronautic Spiders in the Atmosphere. — Mr. Murray states that 

 "a spider's thread, darted through the air,must necessarily acquire electricity 

 from the friction occasioned by its hnpulse through that medium ; and, if 

 propelled counter to a current, the amount of excitement will be greater.'* 

 What would take place under such circumstances, Mr. Murray may amuse 

 himself with supposing, if he pleases ; but, how stands the fact ? Why, I 

 have clearly proved, by direct experiments, given in detail, — experiments 

 requiring no delicate manipulation, which I have invited, and again do invite, 

 Mr. Murray to repeat, that spiders are utterly incapable of darting their 

 lines eve7i through the space of half an inch. These animals, though placed 

 on excellent conductors of electricity, such as metallic rods insulated by 

 water, if exposed to a current of air, let out their lines with facility, and 

 invariably in the direction of the breeze. The act is perfectly voluntary ; and 

 the lines, immediately after they are emitted, nay, at the very time they 

 are issuing from the spinning-apparatus, if blown upon from any other 

 quarter, instantly obey the new impulse thus imparted to them. I have 

 tried this experiment many hundred times, without once perceiving the 

 slightest deviation from these results, which I therefore regard as com- 

 pletely established. To suppose, in direct opposition to the most conclu- 

 sive evidence to the contrary, that spiders, not larger than the head of a 

 small pin, can dart out lines, so fine as to be absolutely invisible, except in 

 a powerful light, to a distance of 10 or 20 ft., through a resisting medium 

 like the atmosphere, implies no ordinary share of mental obliquity ; but 

 to insist, as Mr. Murray does, that they can propel them in a right line, 

 against a stream of air, is in the highest degree unphilosophical. My oppo- 

 nent, anxious to induce an exalted opinion of his merits as an experiment- 

 alist by affecting to despise those of others, insinuates that I am ignorant 

 of phenomena familiar to almost every tyro in electricity, and falls into the 

 absurdity of putting as queries what, in the same paragraph, he assumes to 



* " Upon the iris and the reed ; which renders it probable that their 

 larvae, which are unknown, live in the stems of these plants. According to 

 Linnaeus, the chrysalis of the thick-footed Donacia, which resembles a 

 brown shell, is fixed upon the roots of Phellandrium. 



