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.