^26 



SCIENCE-GOSSIP. 



their aerial journeys, although in captivity they 

 occasionally demonstrate that they are not entirely 

 dependent upon such aids to the successful 

 spreading of their wings. I have known both 

 Dytiscus punctulatus and D. marginalia to escape 

 during the evening from their glass jars and fly 

 from an upstairs room down to the lighted hall 

 below. They make a loud buzzing sound during 

 flight, heard at a considerable distance. 



In common with many other imagines and 

 some few larvae they stridulate when at rest> 

 generally towards nightfall, the notes produced 

 partaking of two very distinct sounds— the 

 one a harsh "shrill," and the other a lower 

 humming or "buzzing" sound, which some 

 authors have associated with the peculiar alulae 

 or winglets situated under the base of the elytra. 

 the use of which is not apparent. The mode of 

 stridulation in the genus can, however, scarcely be 

 said to be satisfactorily solved beyond question. 

 The opinion has been expressed that the - shrills " 

 are generated by the ritbbing of the underside of 

 the elytra against the end of the abdomen (Miall) : 

 but if these areas are examined stridulating organs 

 are not easily perceptible. Mr. C. J. Gahan made 

 no allusion to the Dytisci in his exhaustive paper 

 ••On the Stridulating Organs in the Coleoptera.'' 

 read at a recent meeting of the Entomological 

 Society of London, for the reason, he tells me. that 

 up to that time he had been unable to detect with 

 certainty the presence of stridulating organs in 

 the genus. Since then Mr. Gahan has brought to 

 my notice an article by Hermann Eeeker, " Die 

 Tonapparate der Dytiscida," in which it is stated 

 that stridulation in Pelolius. Dytiscus, and others 

 of the Dytiscidae is generated by rubbing one of 

 the large wing nervures against the elytra. The 

 transverse ridges on these nervures are certainly 

 very noticeable, and according to Eeeker are 

 also more numerous in this family than amongst 

 the generality of the coleoptera. Mr. Gahan. 

 however, calls attention to the fact that many 

 beetles that are not known to stridulate. as well as 

 others which are well known to stridulate in a 

 different way, •■ have the corresponding wing ner- 

 vures just as prominent and as stronglv ribbed 

 across." and he is of Dr. Sharp's opinion (•• Camb. 

 Xat. Hist."') that Reeker's explanation is certainlv 

 erroneous as applied to Pelobins. although it 

 may possibly be correct with regard to Dytiscus. 

 Whether the Dytisci, like the Gyrini or " Whirli- 

 gig Beetles," stridulate previous to flight I do not 

 know, but am personally of opinion that they do. 

 since the cries are almost invariably emitted 

 during the evening, at which time the insects also 

 fly. In captivity, when placed for a change in an 

 ordinary 6-foot bath, transformed by the arrange- 

 ment of water-plants into a natural pool, I have 

 known the males to crawl up a protruding rush 

 and stridulate during the twilight for eight or ten 

 minutes at a time. 



In Dytiscrts the two terminal spiracles are con- 

 siderably larger than in other members of the 

 family Dytiscidae ; this exceptional development 

 being, in fact, one of the distinguishing features 

 of the genus. They are of great service to the 

 beetle in its peculiar method of breathing, for. 

 poising itself, with posterior legs at right angle> 

 to the body and tail upwards at the surface of the 

 water, it is able to bring them at once into direct 

 contact with the atmosphere. In addition to pro- 

 tecting the more delicate under-wings beneath, 

 the elytra also render valuable service by forming 

 a reservoir for air into which the spiracles lead, 

 these being in this genus more dorsally situated 

 than in the majority of the coleoptera (fig. 5). 

 For, although fitting perfectly to a portion of the 

 body, there is left towards the apex, between 

 the flattened back of the insect and the concave 

 wing-covers, a hollow water-tight compartment- 

 which, when stored with fresh air. enables 

 the diver to remain submerged for a considerable 

 period. This reservoir renders the posterior por- 

 tion of the beetle doubly buoyant, so that, if re- 

 maining still and unanchored. being lighter than 

 the water, it rises to the surface tail irpwards. in 

 the right position for aeration. This is effected by 

 slightly raising the wing-covers and first ejecting 

 any used air they may contain. The males are 

 more active and breathe more frequently than the 

 females, a fact easily perceived by keeping speci- 

 mens of both sexes under observation. From 

 experiments carried out by Dr. Sharp, our greatest 

 authority on the carnivorous water-beetles, he 

 found that in Di/tiscus margino.lis the average time 

 passed under water bore to the time the beetles 

 were exposed to the surface for aeration a ratio of 

 about 12 to 1. In less highly developed species 

 the period is very much greater, amounting in the 

 case of Pelobvas Tiernumii, one of the most primi- 

 tive types of the family, to as much as 375 to 1. 



Dr. Sharp's communication to the Linnean 

 Society in 1876 " On the Respiratory Action of the 

 Carnivorous Water-beetles (Dytiscidae) " contains 

 so much interesting information that I cannot 

 refrain from quoting a short summary of his 

 observations bearing on the genus under considera- 

 tion. After giving various details in extenso, he 

 says : — " The male of Dytiscus margvnalis rose on 

 an average once in about 8^ minutes for breathing, 

 and remained on an average about 54 seconds at 

 surface for each respiration. The longest interval 

 it was observed to pass without breathing was 19 

 minutes. The duration of a respiration varied from 

 5 seconds to 300 seconds, and the time it was ex- 

 posed bore to the time it was quiescent a ratio of 



1 : 9j The female D. marginalis rose on the 



average once in about 12| minutes for breathing, 

 and remained on an average about 55§ seconds at 

 surface for each respiration. The longest interval 

 it was observed to pass without breathing was 32§ 

 minutes. The duration of a respiration was from 



