January 4, 1906] 



NA TURE 



! O0 



Eocene volcanic rocks were mostly andesitic flows and 

 breccias; the Miocene wholly andesitic or dacitic, stratified 

 tuffs with coal; the Pliocene a succession of rhyolitic How-. 

 and breccias. 



The description by one geologist of specimens collected 

 bv another at the Antipodes is a division of labour which 

 has drawbacks as well as advantages. Prof. Sollas, how- 

 ever, has faced the difficulty successfully, and the large 

 amount of labour which he has expended on this study 

 has produced results which have a value by no means con- 

 fined to New Zealand geology. Arising from the detailed 

 examination of the rocks, there are a number of more 

 general questions of petrographical interest on which the 

 author is able to throw light. One point is the recog- 

 nition in the ground-mass of the so-called " pilotaxitic " 

 andesites of an interstitial mosaic of quartz, which plays 

 the same part as the glassy base in the " hyalopilitic " 

 type. Another point is the discovery of a certain isotropic 

 hydrated decomposition-product, which partly or wholly 

 1. places the felspar crystals in some of the rhyolites. This 

 was taken by Rutley for glass, and regarded as evidence 

 of the re-fusion of the rock. Our author finds no evidence 

 dI devitrification in the glassy rhyolites of this district, but 

 there may be considerable chemical alteration. The 

 Palaeozoic dyke-rocks are also described and discussed. 

 They range from quartz-diorite, through " dacite-por- 

 phvrite," to dacite, the second being given as a new name 

 to a type of intermediate characters which agrees generally 

 with propylite as defined by Zirkel. We may remark 

 that rhe term " dacite-porphyry " has been used by 

 American petrologists for a rock not essentially different 

 (see, e.g., Iddings in Bull. 150 U.S. Geol. Sur. [1898], 



P- 2 33)- . ,, 



lhe report under notice is marked *' vol. 1., and we 

 may expect that the geological and petrological study of 

 (he Cape Colville district will yield further results of 

 interest. One question which remains to be elucidated is 

 that of the mode of occurrence and origin of the gold. 

 Comparison with the well known Comstock district of 

 Nevada suggests that careful chemical assays of the rocks, 

 both fresh and decomposed, would give significant inform- 

 ation on this point. A. H. 



INSECTS AS CARRIERS OF DISEASE. 1 



" Infinite torment of flies." — Tennyson. 

 'pilE last few years are marked in the annals of medicine 

 ^ by a great increase in our knowledge of certain 

 parasitic diseases, and, above all, in our knowledge of the 

 agency by which the parasites causing the diseases are 

 1 onveyed. 



Chief among these agencies, in carrying the disease- 

 < .uising organisms from infected to uninfected animals, are 

 lhe insects, and, amongst the insects, above all the flies. 

 I lie-, e.g. the common house-fly (Musca domestica), can 

 carry about with them the bacillus of anthrax. Flies, ants, 

 and other even more objectionable insects, are not only 

 capable of disseminating the plague-bacillus from man to 

 man, and possibly from rat to man, but they themselves 

 fall victims to the disease, and perish in great numbers, 

 he] lie active agents in the spread of cholera, and the 

 history of the late war in this country definitely shows that 

 ties play a large part in carrying the bacilli of enteric 

 fever from sources of infection to the food of man, thus 

 spreading the disease. 



lhe diseases already mentioned are caused by bacteria. 

 But flies also play a part in the conveyance of a large 

 number of organisms which are not bacteria, but which, 

 nevertheless, cause disease. 



In considering the part played by flies in disseminating 

 diseases not caused by bacteria, we can neglect all but a 

 very few families, those flies which suck blood having alone 

 any interest in this connection. 



From the point of view of the physician, by far the most 

 important of these families is the Culicidre, with more than 

 300 described species and 5 subfamilies, of which two, the 

 Culicina and the Anophelina, interest us in relation to 

 disease. The gnats or mosquitoes are amongst the most 



1 From an Address delivered before the British Association at Pretoria, 

 l>y A. E. Shipley, F.K.S. 



no. i8S8, vol. 73] 



graceful and most beautiful insects that we know; but 

 they have been judged by their works and undoubtedly are 

 unpopular, and we shall see that this unpopularity is well 

 deserved. Gnats belong both to the genus Culex and to 

 the genus Anopheles. The genus Culex, from which the 

 order takes its name, includes not only our commonest 

 gnat, often seen in swarms on summer evenings, but some 

 hundred and thirty other species. Members of this genus 

 convey from man to man the Filaria noctuma, one of the 

 causes of the widely-spread disease filariasis. In patients 

 suffering from this disease, minute embryonic round-worms 

 swarm in the blood-vessels of the skin during the hours of 

 darkness. Between six and seven in the evening they begin 

 to appear in the superficial blood-vessels, and they increase 

 in number until midnight, when they may occur in such 

 numbers that five or six hundred may be counted in a 

 single drop of blood. After midnight, the swarms begin t.i 

 lessen, and, by breakfast time, about eight or nine in the 

 morning, except for a few strayed revellers, they have 

 disappeared from the superficial circulations, and are hidden 

 away in the larger blood-vessels and in the lungs. 



In spite of their incredible number, some authorities 

 place it at thirty to forty millions in one man, these minute 

 larval organisms, shaped something like a needle pointed 

 at each end, seem to cause little harm. It might be 

 thought that they would traverse the walls of the blood- 

 \essr|- and 1 atis, trouble in the surrounding tissues; but 

 this is prevented by a curious device. Il is well known 

 thai, like inseits, round-worms from time to time casl 

 their skins, and the young larva; in the blood cast theirs, 

 but do not escape from the inside of this winding-sheet ; 

 and thus, though they actively wriggle and coil and uncoil 

 their bodies, their progress is as small, and their struggles 

 as little effective, as are those of a man in a strait- 

 waistcoat. 



One reason of the normal appearance of the creatures 

 in the blood at night is undoubtedly connected with the 

 habits of its second host, the gnat or mosquito. Two 

 species are accused of carrying the Filaria from man 10 

 man — Culex fatigans and Anopheles nigerrimus. Sucked 

 up with thj blood, the round-worms pass into the stomach 

 of the insect. Here they appear to become violent!) 

 excited, and rush from one end to the other of their 

 enveloping sheath, until they succeed in breaking through 

 it. When free, they pierce the walls of the stomach of the 

 mosquito, and come to rest in the great thoracic muscles. 

 Here the Filarias rest for some two or three weeks, grow- 

 ing considerably and developing a mouth and an alimentary 

 canal, thence, when they are sufficiently developed, the) 

 make their way to the proboscis of the mosquito. Here 

 they lie in couples. Exactly how they effect their exit 

 from the mosquito and their entrance into man has not 

 yet been accurately observed ; but presumably it is during 

 the process of biting. Once inside man, they work their 

 way to the lymphatics, and very soon the female begins 

 to pour into the lymph a stream of young embryos, which 

 reach the blood-vessels through the thoracic duct. It is, 

 however, the adults which are the source of all the trouble. 

 They are of considerable size, three or four inches in 

 length ; and their presence, by blocking the channels of 

 the lymphatics, gives rise to a wide range of disease, of 

 which elephantiasis is the most pronounced form. 



We now pass to the second of the diseases carried by 

 gnats, that of Malaria. 



The parasite which causes malaria is a much more lowly 

 organised animal than the Filaria. It is named Ha?ma- 

 mceba, and it too is conveyed by an insect, and, so for as 

 we know, by one genus of mosquito only, the Anopheles. 

 Hence from the point of view of malaria it is important 

 to know whether a district is infected with Culex or 

 Anopheles. The former is rather humpbacked and keeps 

 its body parallel with the surface it is biting, and its larva 

 hangs at an angle below the surface of the water by 

 means of a respiratory tube. Anopheles, on the other 

 hand, carries its body at a sharp angle with the surface 

 upon which it rests, and its larva lies flat below the surface- 

 film and parallel with it. The malarial parasite lives in 

 the blood-cells of man, but at a certain period it breaks 

 up into spores which escape into the fluid of the blood, and 

 it is at this moment that the sufferer feels the access of 

 lever. Their presence and growth within the blood-cells 



