456 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 
Naturelle, &c., vol. ii. p. 324., states that the species of Donacia are generally 
found “ sur Viris et le roseau ; ce qui fait croire que leurs larves, qui ne 
sont pas connues, vivent dans la tige de ces plantes. Selon Linnzeus, la 
nymphe de Ja Donacie crassipéde, quia la forme d'une coque brune, est fixée 
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 Heldodes, 
and the splendid Donaciz, 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 Dyticide, Hydrophilide, 
&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 quércus-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. April3. 1830. 
The Ascent of aéronautic 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 impulse 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 even 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, m the same paragraph, he assumes to 
* “Upon the iris and the reed; which renders it probable that their 
larvee, which are unknown, live in the stems of these plants. According to 
Linneus, the chrysalis of the thick-footed Donacia, which resembles a 
brown shell, is fixed upon the roots of Phellandrium. 
