320 PEECT SLADBN TEUST EXPEDITION. 



Only one who has lived for a long period in a ship or similar situation off the coast of 

 a tropical country can form any idea of the immense numbers of insects which are daily 

 blown off shore by a monsoon wind ; very few of these have any chance of regaining the 

 shore and the vast majority must perish in the sea. Some, indeed, are species of such 

 weak flight that it seems a marvel that they can ever have attained the distances that 

 they do accomplish from the nearest land, and that they do so can only be ascribed to 

 the fact that they are almost entirely wind-borne. 



I am inclined to think that winds form the greatest factor in the ordinary dispersal of 

 most winged insects, if we may use the term " ordinary " in discussing the very slender 

 chances of any individual wind-carried insect happening to hit upon a minute speck of 

 islet in mid-ocean. 



Storms can hardly be considered apart from Prevailing Winds, of which they are often 

 only an exaggeration and to whose effects they are frequently accessory. 



In the case of Christmas Island* we have definite evidence that insects are introduced 

 by winds, but in this particular case these so-called storms appear to be merely an 

 exaggeration of a northerly wind which is more or less normal at that season of the 

 year. 



It appears to me that the real influence of storms on the dispersal of insects consists 

 in an augmentation of the transporting-powers of an ordinary wind, either in carrying an 

 insect a long distance in a very short time (in some of these storms the wind has been 

 recorded at 100-120 or even more miles per hour), or, particularly in the case of cyclonic 

 disturbances, by carrying it high up into the atmosphere, where it may be conveyed by 

 the upper currents of the air for long distances, with very little expenditure of energy 

 on its own behalf, before it finally falls again to the sea-level. 



Marine Currents, although highly important in the conveyance of some other groups 

 of animal life, seem to have played little if any part in the distribution of the Lepidoptera 

 amongst the islands of the Indian Ocean. This seems to be proved by an inspection of 

 the list of species obtained. It will be noted that this list includes no species possessing 

 a female either wholly or partially apterous or even especially lethargic (e. g. Ocneriada;, 

 Fsychidce, &c.). This fact appears to point to the usual method of transmission of 

 species into these islands taking place in the imaginal state rather than in any other. 

 Eor it is noteworthy that all these types, in which the female is normally apterous or 

 lethargic, appear especially favoured for dispersal in the immature condition. Por 

 example, the Ocneriads usually lay an enormous number of eggs, which are not only very 

 often covered with a waterproof varnish, but are generally deposited upon twigs or in the 

 crevices of bark, and whose development in some cases takes place at intervals over long 

 periods of time ; whilst the Psychids possess, in their silken cases and agamogenetic 

 proclivities, an apparently valuable asset against the dangers of the sea. Of course, the 

 disuse and consequent eventual loss of wings after arrival in an island, as in the case of 

 the semi-apterous Tineid moth found in Kerguelen by the 'Challenger' Expedition, 

 belongs to another class of phenomena. 



* ' Monograph of Chriatmas Island,' page 301. 



