122 INDIRECT INJURIES CAUSED BY INSECTS. 



may do no great injury; but where they abound, as they often do, by 

 interrupting the course of the descending sap, and admitting wet be- 

 tween the bark and wood, decay speedily ensues, and the tree perishes. 

 Almost every kind of tree is liable to the assaults of one or more species 

 of this tribe of insects. Even fruit-trees, as the apple, plum, &c., have 

 each their Scolytus; and at Rouen I found a species, I believe unde- 

 scribed, which feeds on the mountain ash. It is to our large forest-trees, 

 however, that they are most injurious. Thus the common ash is assailed 

 by Hylesinus fraxini, the pinnated labyrinths of whose larvae you can 

 hardly fail to observe on the first piece of loose bark you detach from 

 the rough-split posts and rails made of this wood ; while the bark-borer of 

 the oak is a small beetle of an allied genus, Scolytus pygmceus, which with 

 us does no great harm, but so abounded of late years in the Bois de 

 Vincennes, near Paris, that 40,000 trees were killed by it ; and many of 

 the finest elms in St. James's Park and Kensington Gardens 1 , as well as 

 in the promenades of various cities in the north of France, have fallen 

 victims to another of this tribe, Scolytus destructor, whose trivial name well 

 characterises the frequency and severity of its ravages. 2 



1 MacLeay in Edin. Phil. Journ. xi. 123. 



2 While residing at Brussels in the spring of 1836, having pointed out to Dr. 

 George, Professor of Botany at the University, that many of the elms in the park 

 were infested with this insect, and that there was imminent risk of this noble pro- 

 menade, which consists almost wholly of elms, being destroyed by it, he brought the 

 subject under the notice of the burgomaster and municipal council, who very wisely 

 had the diseased trees cut down, as well as the many much younger but equally in- 

 fested trees of the Boulevards, and the bark of the whole peeled off and carefully 

 burnt. I afterwards found, in a tour along the north coast of France through Xor- 

 mandy, &c., that the elms in the promenades (almost always formed of this tree), in 

 all the large towns, were in a course of rapid destruction by this same Scolytus 

 destructor, particularly at Calais, Boulogne, Rouen, Havre, and Caen ; and numerous 

 observations convinced me that the general opinion that these insects attack only 

 those trees which are previously diseased from natural decay is altogether erroneous, 

 and that Professor Audouin's discovery is as important and correct as novel 

 namely, that though it is quite true that the female Scolyti never lay their eggs 

 except in trees which are in a declining state ; yet it is equally certain that the 

 healthiest elms, where Scolyti abound, are constantly brought into this languishing 

 state by the attacks of the males, or, as M. Audouin conceives, of both sexes (see 

 remarks on this point by W. Spence in Trans. Ent. Soc. Lond. ii. proc. xlv.), upon 

 the bark/w/ood; so that in consequence of the loss of sap from the numerous holes 

 which they gnaw, and the subsequent mischief from the rain penetrating into them, 

 the trees are soon brought into that unhealthy condition which the instinct of the 

 female requires to induce her to lay her eggs in them. (Spence in Trans. Ent. 

 Soc. Lond. ii. proc. xiii. xv. xx. xxv. ; Audouin in Ann. Ent. Soc. de France, Bull. 

 Jan.. 4, 1837; Silbermann, Rev. Entom. iv. 115., where Dr. Ratzeburg is quoted as 

 stating that the large weevil (Pissodes notatus) in like manner attacks the bark of 

 young pines with its trunk, and thus renders the trees unhealthy before the female 

 deposits her eggs in them.) For a further description of the mischief done by 

 Scolytus destructor, and the means of preventing its extension, see a communication 

 by W. S. under the article Ulmus, in Mr. London's Arboretum et Fruticetum Britan- 

 nicum ; to which admirable work the reader is also referred for more complete details 

 than could be here given in the valuable contributions by Mr. Westwood relative to 

 insects injurious to this and other species of forest- trees. 



It may be here mentioned, though somewhat out of place, for the purpose of draw- 

 ing the attention of Entomologists to a new tribe of insect -parasites of which no 

 account appears to have been given in books, that in examining closely the pup* of 

 Scolytus destructor at Brussels, I found them lined in different parts of their external 

 surface, but especially on the thorax and about the cases of the elytra, with numer- 



