Januaky 1, 1894.] 



KNOWLEDGE 



Fig. 2. — '"^roli/fus 

 de^stfucfur, or 

 Elm -bark 

 Beetle, magni- 

 fied four dia- 

 meters. 



Fie. .3. — Side view 

 of Elm-bark Beetle. 



The beetle which is responsible for this damage to elm 

 trees is a stoutly and compactly-built shining insect, of 

 somewhat cylindrical outline, averaging about one-fifth of 

 an inch in length (Fig. 2). Its body is black, but the 

 appendages thereto, such as legs, antennae, 

 and wing covers, have more or less of a 

 reddish tinge. The head and thorax are 

 very large and powerful, occupying be- 

 tween them half the length of the entire 

 insect ; and the biting jaws, upon the 

 sti'ength of which the success of the 

 burrowing operations depends, are, of 

 course, stout and very strongly made. 

 The head is furnished above with a pretty 

 clothing of yellowish hairs, which can 

 easily be seen when the insect is viewed in 

 profile, but are not very distinctly visible 

 when it is looked at full-face. Tiut the one 

 feature by which especially it may be recognized and dis- 

 tinguished from all others of similar habits, except those 

 few that belong to its own genus, is the curious shape of 

 the abdomen. This looks as though a slice had been 

 taken off it behind, since it slopes abruptly upwards from 

 a short distance behind the last pair of legs to the ends of 

 the wing covers (Fig. 8). Half a dozen species belonging 

 to the genus Sculytus are known to 

 inhabit Britain ; they are all very much 

 alike both in shape and coloration, but 

 the fact that the various species attack 

 difierent trees is a considerable aid 

 towards their discrimination. The 

 habits of all are similar, though not iden- 

 tical. The family to which .S. drstnictor 

 belongs contains about fifty British species, many of them 

 minute, but all of strong build and cylindrical outline, as 

 befits insects whose sole occupation consists of buirowing 

 in bark. The economy of the group has been much studied 

 by Dr. A. Chapman of Abergavenny, who.se observations 

 have been published in entomological periodicals, and from 

 these some further particulars may now be given. 



The female S. distructur makes preparations to lay her 

 eggs early in .June. She will attack preferably elm trees 

 that have been felled during the previous winter, and Dr. 

 Chapman avers that he has never met with traces of the 

 insect in perfectly healthy growing trees, though, of course, 

 it may be found commonly enough in those that are sickly. 

 The female does not burrow straight inwards at the spot 

 on which she alights ; but, selecting one of the grooves in 

 the bark, she gets to the bottom of this and often widens 

 it for some distance, and then enters the bark itself. The 

 real opening to the tunnel, therefore, lies at some distance 

 from the commencement of the burrows, and hence these 

 entrance orifices are not so easily seen as those by which 

 the beetles make their exit, since these latter run straight 

 outwards from the ends of the galleries and appear like 

 holes carefully made with a bradawl. The female com- 

 mences her labours by herself, before she has received the 

 overtures of a partner, who, in fact, does not put in an 

 appearance till she has already started her internal burrow. 

 But when she has got her burrow well in hand, the 

 expected suitor arrives, or, if he do not, the labour must 

 be stopped to wait for him, as no further progress can be 

 made till he has come. He appears rai-ely to enter the 

 burrow, the toil of excavation being left entirely to his 

 partner. Should the female be so unfortunate as not to be 

 discovered by any member of the opposite sex, the burrow 

 begun in trust will have to be left unfinished and its 

 purpose will never be realized, as no progeny will ever 

 occupy it. But if all goes well, the continuation of the 



burrow can proceed. It is made straight up the trunk for 

 about three inches, and as it is pushed forward eggs are 

 deposited at regular intervals on each side, each in a little 

 depression close to the wood. About a hundred are thus 

 fixed in double row along the tunnel ; they are glutinous 

 as laid, and become coated with a rather thick layer of 

 " frass," as the powdery refuse in the burrows is termed. 



Something like three weeks will have been consumed in 

 the construction and peopling of the central channel, but 

 before the end of that time the eggs first laid have 

 hatched, and the little white footless maggots have begun 

 to make their transverse burrows, side by side, though 

 still not in actual communication, and yet so close as only 

 to leave a very narrow wall between. These burrows are 

 in the softest part of the bark, the cambium layer and 

 adjacent soft tissues being devoured, thougli but very little 

 impression is made on the wood beneath. When the tree 

 is stripped, one can see that while the bark itself contains 

 the greater part of each burrow, one side of each has 

 slightly encroached on the wood, so that the bared trunk 

 shows in feeble outline a pattern corresponding to the 

 typograph which appears deeply cut in intaglio in the bark. 



In about a month's time the larvffi will be full-fed ; most 

 of them will then burrow for about half an inch straight 

 into the wood at the end of their burrows, and then 

 excavate a little chamber, which is to be the winter 

 quarters. Where this communicates with the larval 

 burrow a plug of frass is placed to block up the entrance — 

 a wise precaution, for otherwise, if the bark should fall oft' 

 during the winter, as the damage it has received might 

 well cause it to do, they would be exposed, not only to the 

 severity of the weather, but also to the attacks of in- 

 sectivorous birds ; while if the bark remain on, they will 

 then be doubly protected. Some of the more enterprising 

 ones, however, do not behave thus, but hurry through 

 their changes and come out as perfect beetles in August, 

 piercing to the outside a straight circular burrow at the 

 end of their gallery. What becomes of these individuals 

 is not yet very clear. Of course, those that have burrowed 

 into the wood escape next sprmg in the same way as their 

 predecessors. We thus notice that there are two sets of 

 cylindrical holes made in the bark of the tree — the entrance 

 holes of the parent females, partially filled up and not 

 very conspicuous ; and the exit holes of both males and 

 females of the young brood, which are open and easily 

 seen. There are also two distinct classes of galleries — 

 the vertical one made and occupied by the mother of the 

 family only, and serving as her mortuary chamber when 

 she expires at the close of her labours ; and the lateral 

 galleries made and occupied solely by the larvse. Thus 

 the whole typograph contains, as we said above, the family 

 history of one complete generation, carved in wood, and 

 left as a permanent memorial long after the generation 

 whose adventures it chronicles has passed away. 



The bark of ash trees is often found to be channelled in 

 a somewhat similar way ; but this is easily seen to be the 

 work of a dift'erent insect, as there are some striking 

 dift'erences in the plan of the typograph. We still have 

 the central channel and the host of side channels running 

 at right angles to it, but the actual directions are exactly 

 reversed; the central channel is now horizontal, and the 

 side channels are perpendicular, running above and below 

 the transverse main trunk. ^loreover, the main trimk is 

 composed of two arms which lie nearly m the same straight 

 line, but diverge right and left from an exceedingly short 

 perpendicular stem, the whole forming a figure something 

 like a T of greatly exaggerated breadth and diminished 

 height (Fig. 4). The beetle whose family record is 

 preserved in this form is called Hylcsinux t'ra.vini. It is a 



