KNOWLEDGE. 



[January 1, 1894. 



BARK-BORING BEETLES. 



By E. A. Butler. 



ABOUT two hundred years ago, an English knight, 

 Sir Matthew Dudley by name, planted in his 

 grounds in Northamptonshire a number of young 

 elm trees which were destined to have a more 

 than passing interest. The misfortunes that 

 befell them caused their memory to be perpetuated in the 

 annals of British science by becoming enshrined in the 

 riiilosophical Transactiofis of the then recently formed 

 Royal Society. All went well with the saplings for a time, 

 but one dry summer one of the trees began to show 

 symptoms of ill-health ; its leaves became yellow and 

 began to drop prematurely. As this particular tree stood 

 higher than the rest and in shallower soil. Sir Matthew 

 attributed its ill-health in some measure to the drought, 

 and therefore caused the earth about its roots to be 

 loosened, and two hogsheads of water, together with a 

 quantity of manure, to be supplied to it, by which means 

 the tree was for a time restored to a fairly prosperous 

 condition. But Sir Matthew, being a close observer of 

 nature, had noticed at the same time that on its bark there 

 were " numbers of little black flyes of the beetle kind," and 

 the removal of portions of the bark showed that these 

 beetles had perforated it, and had made tunnelled 

 excavations in its inner layers. He rightly conjectured 

 that the beetles had something to do with the bad 

 condition of the tree, though he regarded the insufficiency 

 of moisture as the primary cause of its unfortunate con- 

 dition. In endeavouring to link together cause and effect, 

 and so to trace the proper sequence of events, he hit upon 

 the idea that the drought had caused the sap of the tree to 

 become thick and syrupy, and that the sap in this condi- 

 tion had formed an attractive bait for the beetles, which 

 had then still further weakened the tree by their burrows. 

 The remedial measures adopted seem to have checked 

 the evil for the time ; but on the advent of another dry 

 summer a few years afterwards, several of the trees again 

 looked sickly, and particularly the one previously attacked. 

 Examination showed that in them all the bark at its 

 attachment to the tree was dry, and that there were " vast 

 numbers of these little Hyes, who had pierced the bark in 

 multitudes of places." Having adopted the same remedies 

 as before. Sir Matthew visited the trees again the next morn- 

 ing, and found that in one case sap was oozing from the bark 

 through the holes pierced by the beetles, and that numbers 

 of flies and wasps were congregated on the trunk, busily 

 engaged in sucking up the sweet liquid. The removal of a 

 little- bark revealed plenty of moisture beneath, and showed 

 that all the burrowing beetles had either gone or been 

 drowned in the flood of sap which had inundated their 

 burrows. This tree was ultimately saved, but the bark of 

 many of the others, which seem to have been more 

 damaged and further advanced in ill-health, remained dry, 

 and the beetles still had things all their own way with 

 them. Being naturally anxious as to the fate of his trees. 

 Sir Matthew, to whom the beetles were e\idently some- 

 thing quite new, carefully examined the bark and found 

 that each beetle had made a " strait perpendicular channel 

 from the entrance upwards, about two inches long, very 

 little if at all bigger than just to move themselves strait- 

 forwards in ' ; for, he adds, " I observed they all of them, 

 if disturbed, came out backwards." Further observation 

 showed that " all along on each side the channel, as close the 

 one to the other as they well could so as yet to be distinct, 

 there were small channels running horizontally from it, 

 in every one of which, at the extremity thereof, was a 

 maggot, in size just the bigness of the small channels. 



very lively, whitish, and almost transparent." He at 

 once recognized that these transverse channels must have 

 had a different origin from that of the central perpen- 

 dicular one (Fig. 1). While that was the work of the 



parent beetle, it 

 was evident that 

 the side branches 

 were excavated 

 by the maggots 

 which were found 

 at their termina- 

 tions ; for they 

 were too narrow 

 to admit of the 

 entrance of the 

 perfect insect. 

 The eggs were 

 evidently laid at 

 regular intervals 

 along the sides 

 of the central 

 gallery, and then 

 the grubs 

 hatched from 

 them began at 

 once to burrow in 

 directions paral- 

 lel to one another 

 and at right 

 angles to the 

 mother's gallery, 

 increasing the 

 diameter of their 

 tunnels as they 

 advanced, to ac- 

 commodate their 

 own enlarged 

 dimensions, and leaving the burrow behind them more or 

 less blocked up with "very small particles which when dry 

 become fine dust," the remnants of food and excrement. 



But the whole life-history of the little pests was not yet 

 fully disclosed, and time was necessary to permit of their 

 proceeding with their metamorphoses. Accordingly we 

 find further observations undertaken a few months later, 

 both upon living trees and upon felled timber stored in the 

 timber yard. The central channel was now found in all 

 cases to contain the dead body of the mother beetle, while 

 the side channels were empty, their occupants having 

 escaped each through a neat circular hole in the bark at the 

 end of its burrow. At the mouth of this hole, however, each 

 had left, as evidence of its former presence, a "whitish, 

 pretty tough skin, exactly the colour and size of the 

 maggot." This, of course, was the chrysalis skin ; so that 

 it was evident that the larvae had matured in their lateral 

 burrows, had there become pupte, and the perfect beetles 

 produced from these had eaten their way into daylight, 

 making a hole in the bark just large enough to admit of 

 their crawling out. Such is the earliest English account 

 of the life-history of that terribly destructive timber-pest, 

 the little beetle now called Scolytus dfstructoi-, which was 

 so ruinous in its effects on the above occasion that scarcely 

 any of the trees were saved. In all essential details these 

 observations have been repeatedly confirmed by sub- 

 sequent entomologists, and few results of insect energy 

 are now better known than these bark-sculptured family 

 records, or " typographs," as they are called, each of which 

 registers the total work of a mother and her entire youthful 

 progeny till the latter have reached maturity, and have left 

 the maternal home to push their own fortunes in the world. 



Fig. 1. — B;ii'k burrowings, or " topograph " of 

 Scohftux (hstriiHor. a, motliei- gallerv ; 

 b, larval galleries. 



