116 



KNOWLEDGE. 



[May, 1908. 



only to the aeronaut sailing at their level, is when a cloud 

 stratum assumes the appearance of long rollers, due 

 apparently to the action of enormous waves of air. Such 

 appearances have been more particularly noted on the 

 Continent, where the rollers have been estimated as 

 measuring a third of a mile from hollow to hollow, and in 

 this strikingly bearing out Professor Helmholtz's theory 

 with regard to the extent of atmos|iln'ric waves. 



It is when a cloud stratum has been subjected for hours 

 to the action of a hot sun that it will begin breaking up 

 into chinks and hollows, which come as a revelation to 

 the aerial voyager. These are seen by him when at a 

 short distance above the cloud level long before any rifts 

 in the cloud ceiling are noticeable from the earth. At 

 first they appear as mere dark pits or clefts ; and, if the 

 sun be low, may scarcely be distinguished from the mere 

 furrows or hollows of the cloud thrown into shade. But 

 attentive watching will presently reveal the fact that 

 brighter objects on the earth — road»vays, water, or white 

 buildings — are flitting past underneath. 



Sometimes, however, it is the observer below whose 

 vision can best gauge and penetrate a cloud. For instance, 

 the density of an attenuated cloudlet is apt to be wrongly 

 estimated by an observer who is immersed within it. Thus 

 a balloon, as observed from the earth, may be only thinly 

 veiled by a cloud which nevertheless completely blots out 

 all objects from the aeronauts themselves. 



It is no uncommon experience when level with a bed of 

 stratus cloud to see cumulus cloud masses rising and 

 rearing themselves into heaps above. A very similar 

 phenomenon, and one of rare beauty, is often to be seen 

 from aloft on overcast days, when perhaps a leaden sky 

 overhangs all the laud. Under such conditions I have 

 found the cloud bed, which I have probably entered before 

 an altitude of "2000 feet has been reached, to be upwards 

 of 1000 feet thick, and the ujiper surface to be of one 

 general level extending as far as the eye can reach. Here 

 and there, however, this general level will be reared into 

 a stately dome without assignable cause, and suggesting 

 the thought that a mountain peak must be beneath. 

 Professor McAdie has obtained some magnificent photo- 

 graphs from Mount Tamalpais of this appearance, which 

 he has designated cloud pyramid, and which he has dis- 

 covered to manifest itself over land which is perfectly 

 level. The phenomenon may occur towards evening, when 

 the upper air is beginning to reach a higher temperature, 

 and the cloud column must be attributed to ascending 

 currents. Instead of assuming the form of dome or 

 pyramid the mass upreared will sometimes simulate the 

 appearance of a vast wave breaking into spray, and attended 

 as it were with veritable " spindrift" borne away on the 

 gale. Under these circumstances, it seems little less than 

 a mystery that such a fugitive and ethereal object can 

 possess so much permanence as to remain practically 

 unaltered in appearance for a long period. 



There are certain days when though the sky may be 

 devoid of actual cloud, it is full of an iudefinite haze 

 which is the ultimate form of cloud. On such days you 

 may ascend, and by the time the first mile has been 

 climbed you may be looking do'ivn on a universal lake of 

 haze, the upper limit of which is as well defined and level 

 as the surface of still water, while overhead the sky is 

 absolutely clear. 



As to the upper clouds which the balloonist in a general 

 way does not reach, it can only be said that from ordinary 

 " cloud level " they appear more clearly defined and nearer 

 than when viewed from earth, not merely because they are 

 actually beheld at closer range ; but also because they are 

 now seen in a transparent sky of purest and deepest 

 blue. 



Conducted by M. I. Cross. 



THE MOUTH PARTS OF THE TSETSE FLY. 

 By W. Wesch^, f.r.m.s. 



[Continued frnm page 92.) 

 There is little doubt that the suctorial mouth parts in the 

 Mucidse were evolved from an ancestor with complete biting 

 mouth parts, and it is curious and interesting to find that this 

 suctorial mouth is again modified iu the Tsetse into a biting, 

 or blood-sucking mouth. But there is no retrogression to the 

 earlier type, the maadibles do not emerge from the upper part, 

 or the maxillse from the lower, but the develo])ment is on fresh 

 hues. The lower lip has become horny, by a simple process 

 of the chin plate, the mriitum, having spread over its membrane, 

 at the same time contracting it, and shrinking it into a slender 

 tube. But at the extremity, instead of the tracheated discs, we 

 find the tip modified into a series of laminated hard ridges, and 

 at the extremity of these ridges the teeth, which in the suctorial 

 mouth were inside and lower down (Figs. 1 and 'I). The reason 

 of these changes is obvious ; it is necessary for the proboscis to 

 become hard and strong in order to penetrate the thick skin of 

 large herbivorous mammals. On examining the tip of the part 

 with higher jiowers the resemblance of the laminated structure 

 to that on the ovipositor of some flower-haunting Hemi])tera 



FlO. 1. — Mouth parts of Tsetse fly {Glo':sina morsitans). (All 

 lateral views.) FlO. 2.— The tip of the proboscis of the Tsetse Hy 

 more magnified, showing cliitinous ridges and teeth. Flo. 3. — Tip 

 of ovipositor of Capsus latiiarus, sho%ving the chitinoua ridges used 

 in piercing stems of plants. Fia. 4. — Mouth parts of Utomoxi/^ 

 calcitrans, L., a very common British blood-sucking fly. FiG. 5. — 

 Mouth parts of Hcsmatobia irritans, L., a British blood-sucking ily. 

 Fig. 6. — Mouth parts of Proscna si/Hrita, F., a British flower- 

 feeding fly. 



is evident. The ovipositor in these insects is used to bore into 

 wood (Fig. 3). On the suctorial mouth there are two transverse 

 levers which expand the discs ; these are very well marked, and 

 obviously homologous in the Tsetse fly. The lancet hypo- 

 ph.arynx) seems to have weakened and the upper li]) to have 

 atrophied. This is probably occasioned by disuse, though in 

 Stomo.ci/x both these organs are present (Fig. 4). The palpi 

 have greatly lengthened, showing that they have important 

 work to do. In our Enghsh Htimalobia iniUins this has also 

 taken place, but in H. siimtilans and Stomoxyn calcitrans we 

 find them of the usual size, or even in a degenerate state (Fig. 5). 

 The proboscis of Prusena syburita is even more like that of 

 the Tsetse than those mentioned, but its use is quite different ; 

 it is used to suck the juices from the long tubes of flowers, but 



