INSECTS. 665 INSECTS. about 3000 species of fossil insects are known, and these are distributed among the geological eras as follows: About 10 per cent, are Paleo- zoic, 15 per cent. Mesozoic, and 75 per cent, have been described from the Tertiary formations. EiDLiOGHAPiTY. Von Zittel and Eastman, Text- hook of Paleontology, vol. i. (London and New York, 1900) ; Zittel and Barrois, Tniilc de paleontologie, part i., vol. ii. (Paris, Munich, and Leipzig, 1887); Scu<lder, "Systematic Re- view of Our Present Knowledge of Fossil In- sects," in Bulletin of the United States Geolog- ical /S'unri/ A'o. 31 (Washington, 1886) ; also "The Tertiary Lake Basin at Florissant, Colo- rado," in Twelfth Annual Report of the Geologic- al and Geoyrajihical Survey of the Territories, Uayden in Charge, for 1878 (Washington, 1883) ; Fosail Insects of North America (2 vols.. New York, 1890) : and other papers by same author in Bulletins of the United States Geological Suriei/. Nos. 09, 71, 93, 101, and 124 (Washing- ton, 1890-95), in Monographs of United States Geological Survey, vols. xxi. and xl. (Washing- ton, 1893-1900). and in Atinual Report of the United States Geological Survey, vol. xiii., part ii. (Washington, 1893). For descriptions and fine figures of the interesting insect fauna of Commentry, France, see Brongniart, Reeherches pour servir I'l I'histoire des insectes fossiles des temps primaires (2 vols., .Saint Eticnne, 1893). See also articles on orders of insects. INSECTS, Propagation of Disease bt. Care- ful observation and patient investigation have resulted in establishing the fact that the trans- mission of disease, in many instances hitherto unexplained, has been due to the agency of insects. The most momentous medical discovery during the year 1899 was that of the cause of malaria in human beings. There are several scientists to whom credit is due in the matter; but the lion's share of it belongs to Patrick Manson. of Dublin, and Major Ross, a surgeon in the British Army, stationed at Calcutta. During several years of laborious work they forged the links that completed the discoveTy. As long ago as 1807, Crawford, an American physician, sug- gested the possibility of the transmission of malaria to man by the mosquito. Again, in 1848, Nott. of New Orleans, referred to the part played bv insects in propagating malarial fever. In l'SS3 A. F. A. King (q.v.), of Washington, D. C, reiterated the theory at some length. Laveran (q.v.). who, in 1880, discovered the pinsmodiinn nialariw. or ha-mosporidium. the parasite of the disease, decl.ared in 1S91 his adherence to the mosquito theory, as did also Fliigge in the same year. In 1892. Pfeiffer showed that certain pni- tozoa, called cocciditi. which are found as para- sites in the rabbit, are capable of two cycles of development, one being exogenous. He mentioned that Koch (q.v.) had suggested that a similar condition might hold good for the parasite of malaria, and that exogenous malarial spores might be conveyed to man by the agency of blood- sucking insects. In 1894 Manson, of Dublin, ap- peared as a vigorous supporter of the mosquito theory as best calculated to explain the various conditions of (he problem. He drew a parallel between the malarial parasite and the frlaria Bancrofti. which he had investigated very thor- oughly. He suggested that the female mosquito fills herself with infected blood, deposits her eggs, and dies beside them. The water in which she lies becomes contaminated with the spores developing in her body, and is then drunk by men, or the spores are inhaled with dust from dried puddles, or the larviE after being hatched feed on the dead body of the mother, and thus become carriers of infection, or the ground may become infected by the bodies of mosquitoes that die and fall upon it. Ross's worl» in his studies of malarial organisms in birds' blood is really monumental. He discovered that after a special variety of gray mosquito had 'fed on the blood of birds containing a certain mature organism called proteosoma, the stomach wall of the insect always contained certain pigmented eorcidia two days later. After other changes had occurred on the eighth or ninth day, the coccidia ruptured and set free innumerable tliread-like bodies which are distributed by the blood-current of the mos- quito through her tissues. Eventually these bodies are found in certain glands in the thorax of the insect, whose ducts open at such a point aa to furnish secretion that lubricates the lancets of the mosquito. When slie punctures the skin of her victim, this secretion, containing the tliread-like bodies, is injected into the bottom of the wound. Manson and his pupil. Daniels, proved that these thread-like bodies, or germinal spores, develop into mature proteosomata in a bird bitten by the infected mosquito, thus com- pleting the cycle. 'hile JIanson was conducting his experiments, Grassi, the distinguished Milan- ese, was completing his studies on the mo.squitoes of Italy. In 1898 he determined that Anopheles claviger was the variety that carried malarial infection. A man who had never been subjected to malarial infection was exposed to the bites of the mosquito Anopheles claviger. and in a short time began to suffer from malarial fever. Qui- nine cured him of the attack. The Italian scien- tists Bignami, Bastianelli. and Grassi, working in conjunction, arrived at the conclusion that the malarial hemisporidia (which consist of small reproductive cells produced within a cyst) run tlirough a cycle in man which is characterized by a long amoeboid stage and an absence of the en- capsulated phase, reproducing themselves a great number of times during the completion of this cycle, and also giving rise to forms which remain sterile in man, known as Grassi's gameti. These latter forms, taken into the stomach of the Ano- pheles with the blood she draws from an in- fected man, develop into sporozoa, and these in turn form sporozoites. delicate filaments which fVnd their way to the salivary glands of the Anoiiheles. The saliva anointing the lancet of the Anopheles carries the infection to the next vic- tim bitten. In East and Central Africa experi- ments were made by Daniels which led him to believe that Anopheles funestus (Giles) is the chief agent in distributing malaria in that coun- try, and that man is the only intermediate host of the malarial parasites. A practical confirmation of the mosquito the- ory resulted from the labors of Low. Celli, Grassi, and the Red Cross of Italy, which demonstrated in 1901 that the use of the mosquito-net and the draining of stagnant pools prevented malaria in individuals living in notoriously malarious coun- tries. Covering the surface of stagnant water with a pellicle of crude petroleum suffices, in many instances, to kill the larv.T of the mosquito and decrease of malaria follows. A solution of
