36 INSECTA MADERENSIA. 



Canarian one. Thus, for instance, no mention is made whatsoever of the pale 

 suture, which (though occasionally ohscure) is never absent from the O. 3Iaderensls : 

 nor can I at all recognise the greatly produced humeral angles of the elytra, and 

 the fiexuose hinder margin of the prothorax, wliich in that description constitute 

 two of the most important features. The elytral strise, likewise, are said to be 

 deep, and the interstices convex, neither of Avhich is the case in those of our 

 present insect, — which woiild appear moreover to be larger than the one there 

 detailed. Hence, I conclude, either that the two are in reality distinct, or else 

 (assimiing M. BriUle's diagnosis to be a generally correct one) that the Canarian 

 form is a very decided variety as compared with the !Madeiran one. Be this how- 

 ever as it may, the Avant of any certain information on the subject renders it not 

 only desu'able, but even necessary not to amalgamate them. The O. Madei'ensis 

 may be at once known from the O. ErlccB by its larger, broader, more ovate, and 

 convexer body, by its darker colour, wider and more posteriorly-rounded pro- 

 thorax, and by the three impressed points on the disk of each of its elytra being 

 smaller and less evident than those of that species. It is usually also more glossy, 

 and its pale elytral margins are sometimes only ajiparent at the shoulders, since 

 the lighter colour has always a tendency to vanish posteriorly. In their liabits 

 the two species are altogether dissunilar, the first occurring, almost exclusively, 

 under stones in open grassy spots, Avhilst the second harbours beneath the bark 

 and fibre of trees -within the sylvan regions. The O. Maderensis, moreover, 

 inhabits a wider extent of country, not only making its appearance at a lower 

 elevation than the O. EriccB, but ranging to a higher one. The former indeed 

 may be said to commence at 2500 feet above the sea, and to continue to the 

 summits of the loftiest peaks ; whereas the latter is not found in any profusion 

 below 4000, and, after passing through its maximum at an even higher level, it 

 almost ceases at an altitude of about 5000 feet. Ovu* present Olisthopus seems to 

 be more particularly abundant from the end of the summer to the following spring, 

 existing in large numbers on most of the grassy mountain slopes and exposed 

 upland districts of the interior of the island, in company with the numerous other 

 insects which delight in such localities. The var. ft, from the Dezerta Grande, is 

 particularly interesting, as not only supplying another example of the results of 

 isolation on external form, but as showing, in addition, the singular tendency 

 which most of the insects cxhiljit on that rock to attain a somewhat larger than 

 the average size. So great indeed is the change which the O. JIadereiisis has 

 undergone, through a long scries of ages, on the Dezerta Grande, that had tlie 

 case been a solitary one, I should not have hesitated in regarding the specimens 

 obtained from thence as specifically distinct ; nevertheless, with the knowledge 

 both of the modifying eff'ects of isolation, and also of the kind of modification 

 essentially ])oculiar to that island, I am perfectly satisfied that it is a mere local 

 state, although a very remarkable one, and has no claim whatsoever to be other- 

 wise considered. 



