General comparative note on the two species.Pediculus humanus (vestimenti) is a larger, more robust and less active insect than P. capitis,—the ♀♀ having a relatively greater egg-carrying capacity than those of the head louse. The eggs are larger and the number laid (under the conditions of these experiments) is greater, while the habits associated with egg laying differ, although placing the ♀♀ of humanus under conditions applicable to capitis or vice versa may induce a considerable degree of uniformity. Cross pairings between the insects are easily brought about and the offspring are fertile inter se. Hybrid strains were maintained until the F. 3 generation and there seemed no reason, judging from breeding results, why such strains should not be continued indefinitely. Nevertheless the marked disparity in the sexes of the F. 1 generation of some of the crosses between P. capitis ♂ and P. humanus ♀ suggests that the parents are specifically distinct.No such obvious disparity occurred between the sexes of the F. 2 and F. 3 hybrid generation, or of either of the pure stocks.Habits. The body louse exhibits some of the habits of a gregarious animal especially during the moulting phases, also a preference for returning to the same spot for oviposition, which leads to the clustering of its eggs. These habits are shown, though in a less marked degree, by P. capitis, and it is possible therefore that they are to some extent the outcome of confinement. Pairing within both species took place at any time during day or night, and was very frequently observed after feeding. ♂♂ with but little food in their alimentary tract were, however, often seen in coitus. The period during which the insects remained paired was frequently observed to be over an hour, but no upper limit was defined.A ♂ of P. humanus fertilized 18 out of 21 ♀♀ placed with him in succession. Four attempts with P. capitis were less successful; one ♂ fertilized ten ♀♀ and very possibly might have equalled the P. humanus record but for a scarcity of virgin ♀♀ while the experiment was in progress. The longest period during which a ♀ of P. humanus retained the power to lay fertile eggs in the absence of a ♂ was 20 days, usually it would seem to be from 16 to 18 days. In the case of P. capitis the period was shorter; 12 days being the longest ascertained period, while it was more usually from seven to eleven days.The greatest number of eggs laid by any one ♀ of P. humanus was 295, an average of 6·4 per day—the daily average of a number of ♀♀ being 5·1. P. capitis ♀♀ showed a lower fecundity, the highest record being 141 with a daily average of 4—the general average being 3·7. These figures are probably exceeded under natural conditions. An experiment in differential feeding with P. humanus (Table VIII) shows clearly that fecundity is dependent on feeding. When extra feeding time over and above seven hours per day was given the average for four ♀♀ was eight per day. It is reasonable to suppose that the average for P. capitis would also be increased by unrestricted feeding.The fertility of the eggs laid was not affected by increased feeding. The greatest number of fertilized eggs laid by a ♀ P. humanus after the removal of the ♂ was 115 (♀ No. 9), with a ♀ showing a higher daily laying average this might well be exceeded. With P. capitis the parallel figure is 70 (♀ No. 9). The ♀♀ of both species, after arriving at maturity, started oviposition irrespective of their having paired or not, but eggs laid by virgin ♀♀ were invariably infertile.Length of life. The life of the ♂ P. humanus used in the experiment recorded in Table I was 32 days; the longest ♀ life was 46 days, with an average of 34. For P. capitis the figures were: ♂ life 30 days; ♀ life 38 days, with an average of 27 days. Whether or not the average lives of the insects would be extended by unrestricted feeding is an open question.The life of the hybrid insects was not noticeably shorter than that usual for P. humanus, and they seemed to thrive better than P. capitis.Tests made with unfed P. humanus showed that the longest lives were at a medium temperature of 16° to 18° C., many of the insects living from three to four days, while two lived five and one lived seven days. At 24·5° C. all died within five days. At 36·1° C. all died within three days.Newly-hatched larvae, unless fed, lived less than 24 hours at 36·1° C., and when kept in a box in the vest pocket they lived but little more than a day; none survived a second day.Adults kept in a box in the side pocket of a coat lived five days without food; this was in March.Moulting. 40 young lice were reared in a box carried in a vest pocket and particulars of their moulting recorded.1st moult: 3% moulted on the 3rd day; 42% on the 4th and 55% on the 5th day.2nd moult: 15% moulted on the 7th day; 72% on the 8th and 13% on the 9th day.3rd moult: 5% moulted on the JOth day; 3% on the 11th, 55% on the 12th, 32% on the 13th day, while 5% took 14 days to reach maturity.The ♂♂ usually mature rather earlier than the ♀♀Cold. Active specimens of P. humanus survived two days at a temperature of −2·3° C. to −1·1° C., but none recovered after exposure to these conditions for a week.Hatching of eggs. Table IX shows that under humid conditions at 31° C. 3% of the 1300 eggs tested hatched on the 7th day; 56% on the 8th; 33% on the 9th; 8% on the 10th and ·2% later on the same day or on the 11th.A test of batches of eggs taken from a stock box, some of which must have been laid several days previously, showed that none hatched at 15·6°−18·4° C., while at 24·5° C. there was considerable egg mortality, and the hatching period was spread over a longer period than usual, though not to the extent mentioned by Warburtou (1909); at 36·l° C. hatching was spread over five days and the mortality was not excessive.To give some idea of the possible rate of multiplication of P. humanus we may estimate the egg period as 12 days and a further 12 days to the maturity of the ♀♀. Allowing an average of eight eggs per day, spread over a fertility period of 40 days, we find that, during her life, a single ♀ may have 4160 offspring.
THE rapid fall in the number of plague cases with the onset of hot weather is a characteristic feature of plague epidemics in the northern half of India. In these regions the rise in temperature is accompanied by an increased drying capacity of the atmosphere so that it is impossible to assess to what extent the higher temperature and increased drying power are respectively responsible for the effect.Epidemiological observations made in Mauritius (1899), Southern India (1908) and Java (1914) suggest that a rise of temperature, without increased drying power of the air, restrains plague epidemics. These observations have been ably analysed by Brooks (1917). The object of the experiments set forth below was to ascertain the separate influences of temperature and drying upon the longevity of fleas with a view to the interpretation of the epidemiological facts.To save possibility of confusion we may be permitted to point out that the meteorological term relative humidity does not indicate the drying capacity of an atmosphere. Drying power depends on the saturation deficiency, that is the extent to which the vapour pressure of water in the air is short of the saturation pressure for the particular temperature. Investigation both by members of the Plague Commission in India (1912) and by Bacot (1914) in this country have shown that considerable saturation deficiency exerts an inimical influence at almost every stage of the life history of the insect and the consequent diminution in flea-population no doubt plays a considerable part in modifying the spread of plague. Our observations are only concerned with the effect upon the longevity of the adult insect apart from its host.Whilst living amongst the fur of an animal the insect is exposed to a nearly constant climate. Also, it can slake its thirst whenever it is so disposed. Once separated from its host, however, the time which elapses before its career is ended by desiccation is principally determined by the drying power of the atmosphere. This is abundantly illustrated by the experiments in the reports referred to above and also by the observations of Nicoll (1912) and of Petrie (1923) in Egypt.Apart from the general reduction in flea population, saturation deficiency 1 The experiments recorded in this paper were made in 1914. The work was put aside during the war. For various reasons, amongst them the death of Arthur Bacot, who contracted typhus whilst experimenting with infected lice, the observations have not been recorded until now.
Fleas are insects that undergo a complete metamorphosis in the course of their development from egg to adult. The eggs laid by the mother are not, as are the eggs of lice, attached in any way to the skin, fur or feathers of the animal on which the parents are parasitic. They fall into the nest, or drop to the ground within the lair or “ run ” of the host. Hatching takes from three to ten days according to the temperature. The larvae are active, whitish maggots, eyeless and legless, which are not parasitic, but feed on organic matter in the host's bed, or in the dust that collects on the ground in its proximity.
These experiments were performed in response to a question submitted to me from the Royal Sanitary Institute. The point at issue was the possibility of eggs of the common bed-bug, Cimex lectularius, surviving the process of house-destruction, when the plaster from old walls, on which eggs had been laid, was broken down and remixed with fresh mortar for making the partitions of rooms in new tenements; such survival having been given as an explanation for previously unoccupied houses being infested with bugs. , Methods.The ova used in the following tests were obtained by placing twenty to thirty adult specimens of C lectularius in a lj-inch glass-bottomed entomological box. The open top of the box was covered with fine gauze, through which the bugs were allowed to feed liberally each day. When feeding was not in progress the box containing the insects was kept in an incubator at 75°F. A slip of cloth was used as a lining to the box to afford foothold. As the great majority of the eggs were laid on the cloth this plan afforded an easy means of dividing up batches of eggs into two or more lots, so that controls could be obtained without any disturbance to the eggs. In order to get large numbers for some of the experiments the eggs were in certain instances allowed to accumulate for from three to five days before removal.In experiments in which the eggs were submerged in water or buried in sand, those laid on cloth alone were used, but where the test was one of temperature alone, eggs laid on the sides of the boxes were occasionally utilised. In the case of the experiment with plaster the eggs were carefully detached from the sides of the box and cloth on which they had been laid. Control eggs were always taken from the same batch as those experimented with and all control eggs were kept from the time of laying in an incubator at 75°F.The attached table shows the number of eggs in each batch, together with the date of laying. Batches are lettered from (a) to (r), and as the batch letter is quoted against each experiment, it is possible to follow the history and find the percentage hatching in different portions of the same batch which were subjected to different conditions.
The forms under consideration resemble Rickettsia prowazeki—the supposed cause of typhus fever—which occurs in lice that have fed on typhus fever patients.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.