July 1, 1893.] 



KNOWLEDGE 



131 



is often the case that the most insignificant and uninter- 

 esting-looking insects are just those that have the pro- 

 foundest influence on the world around them, and are the 

 most concerned in adjusting the relations between organic 

 and inorganic nature. Few better exemplifications of this 

 could be found than in these apparently uninteresting and 

 fragile flies. Thoughts puch as these, of the great variety 

 of insect life and the wide-reaching power of little things, 

 constantly rite before the mind as one reads this account 

 of the first half of the British Nematocera, or "thread- 

 horned flies." Thus, in the fleas, we have excellent 

 instances of the animal parasite and all the annoyances 

 occasioned by such habits ; and yet the parasitism is 

 imperfect, the insects living upon the blood of their victims 

 only in their adult condition, while in their larval stage 

 they are non-parasitic, and subsist on solid animal matters. 

 Their wingless condition, too, no doubt connected with 

 their parasitic habits, cannot but excite interest and stimu- 

 late speculation. In the next family we get into totally 

 different surroundings. There we find the gall-gnats, 

 which are associated with the vegetable world, and imitate 

 the habits of the hymenopterous ' 'ynipidit by forming 

 excrescences, beautiful or disfiguring, on various plants, 

 and more or less interfere with their economy, by arresting 

 their growth or rendering their reproductive function 

 abortive. Amongst these are not merely the little gnats 

 that produce the hairy galls so common on nettles, the 

 rose-shaped galls on willows, the tufts on white-thorn 

 branches, the swellings on the stems of aspen leaves, and 

 many other more or less harmless monstrosities, but also 

 such serious pests as the wheat midge and the notorious 

 Hessian fly, which are so destructive to cereals. Amongst 

 this same group, also, we find that most remarkable of 

 phenomena, asexual generation, and that, too, not merely 

 by an unfertilized adult, but even by a larva, the stage 

 which is usually totally unconnected with the functions of 

 reproduction. In the fungus gnats, so called from the 

 food of the larviB, we have the remarkable and utterly 

 unexplained phenomenon of the migration of vast hosts of 

 the maggot-like larvae in one continuous line " from twelve 

 to fourteen feet in length, two or three inches broad, and 

 half an inch thick, containing countless numbers, as close 

 together as they can be packed." To this long worm-like 

 composite body, which looks like a thin grey snake slowly 

 working its way over the ground at the rate of something 

 under an inch per minute, has been given the name of 

 " army-worm." In the IJihidnida we have the so-called 

 fever-fly, which does damage to the hop-gardens, as well 

 as the black, sluggish, sprawling flies that often appear 

 in countless numbers in our streets for a few days in 

 spring or early summer, and then vanish as suddenly 

 as they appeared. Perhaps the best known of these 

 are the St. Mark's and St. John's flies, named from 

 the regularity of their appearance about the time of the 

 days sacred to those saints in the calendar. In the 

 Siiiiulidce, or sandflies, we pass into quite other surround- 

 ings again ; for here we come across small flies which 

 persecute man and beast after the fashion of mosquitoes. 

 Here, too, we are introduced to the curious phenomenon 

 of the assembling of the male flies in large numbers, to fly 

 in circles round and round and execute the airy " dances " 

 which are so fascinating to watch. And lastly, amongst 

 the C/iiriiniiwi(he, or midges, another set of dancers, we 

 have the frequent inhabitants of the water-butt, the red, 

 wriggling, aquatic larv;e called " bloodworms," which are 

 incipient house builders, inhabiting, during their moments 

 of leisure, tubes of their own construction at the bottom of 

 the water. Their beautifully plumed locomotive pupje, 

 which are such familiar objects in ponds in spring, are still 



more attractive, and nothing can be more beautiful than 

 the delicate tracery of their cast skins as they float at the 

 surface of the pond with their exquisite white rosettes of 

 breathing hairs. This again is a group which, like the 

 gall-gnats, has got into irregularities over their repro- 

 ductive processes, and exhibits the phenomenon of par- 

 thenogenesis. We have, perhaps, said enough to show 

 that this volume deals with subjects of much and varied 

 interest, and to induce some to follow I\Ir. Theobald's 

 guidance in undertaking the study of the group. Very 

 much yet remains to be done before anything like finality 

 is reached in our knowledge of British gnats, so that there 

 are abundant prospects of reward in the discovery of new 

 facts to anyone who has the time and patience to devote to 

 what, it must he confessed, is a somewhat intricate study. 

 A copious bibliography has been added for the benefit of 

 the earnest student, and there are a number of illustrations 

 explanatory of structural and anatomical points, as well as 

 showing typical species. A little more care might usefully 

 have been expended on " editing." There is a list of 

 errata, but this might have been considerably augmented, 

 there being many misprints not recognized therein, especially 

 in the scientific names. 



Science Notes. 



Sir Joseph Hooker is publishing, with the aid of the 

 stafi" of the Herbarium at Kew, an index to the names of 

 a^l flowering plants. It will be published in four quarto 

 volumes, and will be entitled " The Index Kewensis." 

 Sir Joseph, in giving an account of the origin of this 

 important work, says : " Shortly before his death Mr. 

 Darwin informed me of his intention to devote a con- 

 siderable sum in aid or furtherance of some work of 

 utility to biological science : and to provide for its com- 

 pletion, should this not be accomplished during his 

 lifetime. He further informed me that the difBculties he 

 had experienced in accurately designating the many plants 

 which he had studied, and ascertaining their native 

 countries, had suggested to him the compilation of an 

 index to the names and authorities of all known flowering 

 plmts and their countries, as a work of supreme impor- 

 tance to students of systematic and geographical botany, 

 and to horticulturists, and as a fitting object of the 

 fulfilment of his intentions. I have only to add that, at 

 his request, I undertook to direct and supervise such a 

 work." 



In a recent number of the Km- Bulletin we have a 

 remarkable illustration of the influence of mankind upon 

 local climates. In Africa, the country between the Nile 

 and the Bed Sea is notable for its general barrenness, 

 liut the present condition of things has probably been 

 brought about by human agency. In fact, this is largely 

 proved by the names of localities. Thus, the Arabic names 

 of the valleys are those denoting the names of trees, 

 although now not a single tree is to be met with. It is 

 suggested that the Arab and his camel have been the means 

 of converting a wooded country into a waste. The camels 

 eat the leaves and shoots of the trees, and the Arabs have 

 burned the trees for charcoal. 



At the Royal Society soiree on the 7th June, Captain 

 McEnvoy exhibited the hj^drophone, which, in connection 

 with a new instrument named a kinesiscope, is intended 

 to be used at night, or in foggy weather. It has for its 

 object the prevention of surprise attacks from torpedo 

 boats, or other hostile vessels approaching. It will give 



