SECTIONAL TRANSACTIONS— D. 335 



has a forward propulsive component. In some cases the moving blades 

 are recognisable as such, e.g. the wings of insects, bvit in other cases the 

 whole surface of the body acts as a series of inclined blades which move in 

 a definite rhythm and which work reciprocally across the line of forward 

 movement of the organism . The mechanical principle involved is essentially 

 that of the screw-propeller, although the movements are reciprocating 

 instead of rotary. This type of mechanism is very widely spread through- 

 out the animal kingdom and is particularly obvious in fish, although the 

 movements are often sufficiently fast to make photographic records essen- 

 tial for accurate study. Essentially the same type of movement is found 

 in spirochaetes, nematodes, and in many aquatic vertebrates, where the 

 reciprocating action of each part of the body is effected by the muscular 

 waves which pass along the body. A study of these waves shows that their 

 form is closely correlated with the degree of resistance offered to the 

 water by the body of the organism. 



Contrary to often repeated statements, the tail fin of most fish is orily of 

 value as a propeller when the fish is moving at extremely low speeds ; in all 

 normal movements the tail fin plays a negligible role as part of the propulsive 

 mechanism ; it is more usually a rudder. 



Friday, September 2. 



Joint Discussion with Section K (Botany, q.v.) on Biological balance in 

 fresh water. 



Afternoon. 



Dr. R. H. LE Pelley. — Lygus simonyi Reut., causing abortion of coffee 

 flower buds, and the problem of its control. 



Dr. A. E. Cameron. — The rearing of Hcematopota pliivialis L. {the 

 Cleg, Tabanidce) under controlled experimental conditions. 



Investigation of the cleg, the most common palaearctic species of 

 European tabanid, has been almost entirely neglected, despite its potential 

 importance in the transmission of parasitic micro-organisms of cattle, as 

 for example Trypanosoma theileri. Hitherto the number of larval stages 

 has not been ascertained in any European tabanid. In the course of our 

 experiments adult females were fed on human and rabbit hosts, and they 

 deposited typical tabanid egg-masses. The larvae emerging from the eggs 

 were reared to the adult stage. Several females partook of a second blood 

 meal and oviposited a second time. The fact that two acts of oviposition 

 can occur separated by an interval, during which a second host is attacked, 

 is of prime significance relative to the biological transmission of micro- 

 organisms from one host to another. There is evidence that triplicate 

 oviposition may also occur. 



The number of ecdyses of larvae hatched from the same batch of eggs, 

 and submitted to the same conditions of rearing in all cases, varied 

 from seven to nine. Some were univohine and others demivoltine, even 

 from the same egg-batches. It would thus appear as if H. pliivialis were 

 heterozygous for univoltine or demivoltine characters. 



The earlier ecdyses are readily overlooked, but the risk of their omission 

 has been reduced by a series of measurements of a standard skeletal 

 structure— the paired tentorial rods of the larval head capsule— in all 



