1383

ARBORETUM AND ERUT1CUTUM.

PART HI.

course at about right angles from theprimary channel, on each side of it.

(Seeyfg. 1236.) The true food of theinsect is the inner bark; and theerosion of the soft wood is so slight,as to be, perhaps, nearly accidental.

The course of each individual larva,on each side of the primary channel, isabout parallel to that of the larva nextto it; and each forms a channel by itsfeeding that is enlarged as the larvaincreases in size. When each larva hasfinished its course of feeding it stops inits progress, turns to a pupa, and then toa beetle; after which it gnaws a straighthole through the bark, and comes out.

The beetles begin to come out inabout the latter end of May of the yearfollowing that in which the eggs weredeposited. The sexes afterwards pair,and the females, bearing eggs, borethrough the bark, as before detailed; and so on from generation to generation,and year to year.

The result of the erosions of the female parent, and of the larva, in the innerbark and soft wood, is that of cutting off' the vital connexion between thesetwo parts; and, when the erosions effected in a tree have become numerous, ofoccasioning its death, by preventing the ascent and descent of the sap. Ithas been said that the scolytus never attacks a tree in a perfectly healthystate; and,also, that trees suffering under carcinoma (see p. 1385.) are par-ticularly liable to it. In the year 1825, an avenue of elm trees in CamberwellGrove were attacked by this disease, which was supposed to be brought onby the gas which escaped from the pipes laid down along the road beingabsorbed by the roots; and which gave rise to a suit in Chancery between theinhabitants and the proprietors of the gas- works. Various persons, consideredas competent judges, were employed to ascertain the cause of the decay of theelms; and their general conclusion was, that the carcinoma had been broughton by old age, excavations for building in an exceedingly dry soil, and an extraor-dinarily dry summer, and that the gas had had no influence in producing the decayof the trees. The trunks of the trees, when examined in 1826, were foundinfested with an immense number of larvae feeding on the soft inner bark. Aninteresting account of the Camberwell elms will be found in the Gardener's Ma-gazine, vol. i. p. 378. In relation to the capability of the scolytus to effect injuryon elm trees, it is stated that 80,000 have been found in a single tree. Ithas also been remarked that the scolyti seldom destroy the trees they attackthe first year that they commence their ravages; and that they prefer a treethat they have already begun to devour, to a young and vigorous tree.(See the observations of Mr. Spence in p. 1389.) It is easy to ascer-tain the infested trees, as the bark will be found perforated by small boles,as if made by shot or a brad-awl, in various parts; and little particlesof a substance like fine sawdust will be found on the rough surface of thebark, and at the foot of the tree. The scolyti, as Mr. Denson, sen., hasobserved, never attack dead trees. The Scolytus destructor, as an enemy toelm trees, appears first to have attracted the attention of entomologists n>England about the year 1824, by M‘Leay’s Report to the Treasury upo nthe state of the elms in St. James’s and Hyde Parks. (See this Report inEdin. Phil. Journ., No. xxxi. art. 12.; and see Tilloch ’s Phil. Mag., Oct. 1823,art. 51.) In the year 1828, a controversy was carried on in a Cambridgenewspaper, between Mr. John Denson, sen., the author of A Peasant's Voiceto Landowners, &c., and Mr. J. Deck of Cambridge, respecting the cause o!

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