250 



THE AGRICQLTURAL NEWS. 



Aug VST 5, 1911. 



INSECT NOTES. 



THE POLLINATION OF YUCCA 

 PLOWBRS. 



In the last number of the Agricultiiral News, \). 234, 

 there appeared a briet account of the manner in which the polli- 

 nation of the flower of an Aroid plant is accomplished by 

 small beetles, and of the part played by a fungus in aiding the 

 insects to gain an entrance to the interior of the closed floral 

 envelope. The matter of tlie present article deals with polli- 

 nation of another kind, by insects, and it is intended to follow 

 this by a third article, treating of a further variation in 

 insect pollination. 



In the following notes, the action of a small moth in 

 accomplishing the pollination of a flower of an entirely dif- 

 ferent structure from that of the Aroids will be explained. 

 The flower referred to in this connexion is that of the Spanish 

 needle or Adam's needle {Yucca aloifolia), which occurs 

 throughout the West Indies. The structure of the flovver is 

 similar to that of all in the Lily Family, to which the Yuccas 

 belong. The fertilization of the Yucca flower was first studied 

 and described in detail by Professor C. V. rliley, Entomolo- 

 gist to the United States Department of Agriculture, who 

 published in Insect Life {Vol. IV, p. 358) an article entitled 

 Some Interrelations of Plants and Insects, which included 

 a most interesting account of this process. Since tliat time 

 the observations of Professor lliley have been confirmed 

 by other entomologists, and numerous references to these have 

 appeared in entomological literature. 



The lily-like flower of the Yucca consists of three sepals, 

 three petals, six stamens and a pistil. The stamens are some- 

 what recurved, the anthers being ihus considerably removed 

 from the stigma, making self-fertilization of the flower diffi- 

 cult, or even impossible. The pistil is composed of three 

 carpels, united in such a manner that the stigmas are situated 

 on the inner surface of a central tube, at the top or outer 

 end of the style. 



The pollen produced by the anthers is moist and heavy, 

 and this, taken in connexion with the structure of the stigma 

 and the relative positions of the pistil and stamens, would 

 indicate the necessity for the action of some in.sect or other 

 agency in accomplishing the transference of the pollen. 



The species of Vucca (T. aloifolia) already mentioned 

 depends for the pollination of its flowers upon the voluntary 

 action of a small whitish moth {I'rtmu/ia i/iiccojidla, Kiley), 

 •while other species of this genus of plants arc dependent on 

 other species of insects of the genus Pronuba, 



FronuOa i/uccasella is le.ss than i inch in length, with 

 a spread of wings of about 1 inch. The fore wings are white, 

 but the hind wings and under parts of the body are dusky. 

 The male and female moths are very much alike in general 

 appearance; the female, however, possesses certain structural 

 peculiarities, not to be found in other moth.s, whidi are special 

 developments for the sole purpose of adapting this insect to 

 its relationship to the Vucca plant. These are a piercing 

 ovipositor, by means of which the eggs are inserted into the 

 ovary at the ba.se of the pistil, and a pair of long, hairy pro- 

 cesses developed on the modified mouth parts, which are used 

 for collecting and rarrying pollen. 



The female Pronuba ascends to the summit of a stamen, 

 where, by means of the prehensile organs just mentioned^ and 

 her long, slender tongue, she collects a mass of pollen. It is 

 often necessary to visit several anthers before a sufficient 

 quantity of pollen has been gathered. This is shaped with the 

 aid of the fiont legs, into a small rounded [)ellet, often two 



or three times as large as the head of the moth. Having 

 accumulated a satisfactory amount of pollen, the female moth 

 flies with it to the pistil, generally of another flower, on 

 the same plant, than that from w-hich the pollen was gathered, 

 or to the flowers on another plant, and proceeds to insert 

 the pellet into the stigmatic tube. The fertilization of this 

 flower being thus ensured, she makes her way to a position 

 where the ovipositor may be inserted in such a manner that 

 the eggs will be deposited in the ovary near the ovules. The 

 larvae will then be in the best possible situation to begin 

 feeding on the developing seeds. 



In the article mentioned above, Riley makes the follow- 

 ing statements: — 



'The absolute need of Pronuba in the pollination of our 

 dehiscent Yuccas I have proved over and over again in many 

 ways. The plant never produces seed where Pronuba does 

 not exist; it never produces seed when she is excluded 

 artificially, and experiments which I have made with artifi- 

 cial or brush pollination all show that it is miich more 

 difficult to ensure complete fructification than would at first 

 appear, and that the act of pollination is rarely performed 

 with a brush 'ir by using the flower's own filaments as success- 

 fully as it is done by Pronuba.' 



This remarkable action of the female Pronuba would 

 almost seem to be the result of a process of reasoning. The 

 larvae depend for food on the developing seed.s, but if the 

 flower were not fertilized no seeds would be developed. When 

 the eggs hatch, the parent moth proceeds to pollinate the 

 flower, and then deposits her eggs almost, or quite, in contact 

 with the ovules. ' 



The relation between Pronuba and the flowers of Yucca is 

 n\ade all the more wonderful from the fact that the adult female 

 derives no benefit by it, since the mouth parts and diges- 

 tive system are so modified that it is impossible for her to 

 feed at all. The entire operation appears to be a definite 

 piovision for the development of the young, without any 

 incentive arising from lienefits received l)y the individual 

 performing the act. 



The plant, however, benefits by the action of Pronuba 

 in that it is enabled to produce seeds; the developing larvae 

 do not consume all the seeds formed in the ovary, so that 

 many are left for its propagation. 



It is staled in the London yim.es that the regulations 

 issued by the Belgian Government for the prevention and cure 

 of sleeping sickness in the Congo provide heavy penalties for 

 neglect of the prescribed precautions. All employers of 

 native labour must take measures to discover any cases of 

 sleeping sickness among their statt" and report them at once 

 to the authorities. Those aiding others to neglect the treat- 

 ment prescribed will be punished, as well as those who try 

 to pass from infected to uninfected districts or vice versa. 

 It is noted that in order to combat the disease ettcctively, it 

 is all important to discover those victims who have not 

 yet reached the second stage — somnolence. Such a measure 

 would tend not only to decrea.se the mortality but also to 

 limit the dissemination of the germs. All suspeci.s, therefore, 

 are to be examined by the heads of training posts or sent for 

 inspection to the nearest doctor, who will carry out 

 a thorough examination. Inspection posts are to be established 

 on the main lines of communication in order to prevent 

 suspects from carrying the disease into provinces which are 

 as yet untouched. Natives from the surrounding countries 

 will only be permitted to enter the unaffectcil regions of the 

 Belgian colony after undergoing a searching medical examina- 

 tion at Ala or .Jakoma. (Science, Vol. .\.\'.VII1, p. 768.) 



