Diapause should not be confused with simple inhibition of growth due to cold.

The embryo of the grasshopper, Austroicetes cruciata, enters a state of diapause a few days after the egg is laid. The diapause is obligate like that found in the eggs of Circotettix, rather than facultative like that found in the larvae of Platyedra. These types of diapause should be regarded as two extremes of the one phenomenon rather than two distinct phenomena.

Diapause was eliminated when the egg was exposed to adequate low temperature for an adequate period when the embryo was at a suitable stage of development. If the period at low temperature was inadequate, or if the embryo was not at a suitable stage of development when the exposure to low temperature occurred, diapause was only partially eliminated. In such cases, the exposure to low temperature resulted in an abnormal growth by the embryo when the egg was returned to a favourable high temperature, but development was not completed.

The influence of low temperature on the elimination of diapause was due chiefly to changes which occurred in the yolk during exposure to low temperature. Thus yolk which had been modified in this way became suitable for the nourishment of the embryo, and it no longer obstructed katatrepsis. This explanation of the nature of the obligate diapause in the eggs of Austroicetes brings this phenomenon into line with the facultative diapause in the larvae of Platyedra, which has been shown to be associated with the nourishment of the larva.

Diapause was eliminated most rapidly at temperatures between 6°C. and 13°C. The embryo grew most rapidly at about 30°C. ; below 13°C. growth practically ceased. The absence of overlapping of the temperature ranges for these two processes may explain the presence of diapause in Austroicetes. The occurrence of a less pronounced diapause in the eggs of Melanoplus may be due to a partial overlapping of the temperature range for the two processes. Similarly the absence of diapause in the eggs of Locusta may be explained on the assumption that in this species the two processes proceed over the same temperature range.

An outbreak of body lice might require several control measures according to circumstances. Two types of fumigant action have been investigated in the laboratory and the field :—

(1) House fumigation for disinfesting typhus-contaminated premises.

Lice were fairly similar to bed-bugs in their resistance to the fumigants tested. In practice, 16 oz./l,000 cubic feet of hydrogen cyanide will kill lice and eggs : 2 lb. of sulphur/1,000 cubic feet is also successful if the wind is not strong.

(2) Bin fumigation of garments by saturated vapours.

The saturated vapours of solids such as naphthalene and paradichlorbenzene are extremely slow in action ; even with 20 hours exposure at 20°C. they are unreliable. Liquids such as heavy naphtha or trichlorethylene are much more promising.

For reading and criticising this paper I am grateful to Professor Buxton in whose department the work was done. My thanks are also due to the Medical Research Council for a grant.

1. These studies were originally intended to form the basis of a world monograph of the Siricidae ; such a work cannot be completed under present circumstances.

2. Two ratios are introduced as useful characters for separating species : the ovipositor/forewing ratio and the sawsheath/ovipositor ratio. These ratios were obtained from all specimens of all the species represented in the British Museum collections and the results are tabulated. They were found not to vary with the size of the insects.

The former ratio is specially useful in the genus Urocerus, which has a long ovipositor, and the latter ratio in the genus Sirex, which has a shorter ovipositor.

3. Keys are given to the genera of the world. Of Semenov's new genera, Xoanon is accepted but not Xanthosirex. A new genus Eriotremex is erected for certain Indo-Malayan species previously included in Tremex.

4. Keys are given to the European species, which are compared critically with related species from other parts of the world. A key to the species of Eriotremex, gen. nov., is also given.

5. Sirex noctilio, F., and S. juvencus, L., are recorded for the first time from North America and S. cyaneus, F., from the continent of Europe. The common Urocerus of the northern Palaearctic region is shown to be more closely related to the Nearctic U. gigas flavicornis, F., than to the central European U. gigas gigas, L., and is treated as a new subspecies—U. gigas taiganus, subsp. nov. U. gigas tibetanus, subsp. nov., is described from the Himalayas. U. sah, Mocsáry, is treated as a subspecies of U. augur, Klug, and U. cedrorum, Smith, as a synonym of U. augur augur, Klug.

6. The British Siricidae are discussed, and it is suggested that U. gigas taiganus, subsp. nov., and the form of S. juvencus, L., with entirely black antennae may be native in the Caledonian forest.

7. It is argued that modern Siricidae could not have been derived from the Jurassic Pseudosiricidae.

8. The known Oriental and Himalayan Siricidae are listed and discussed. Urocerus multifasciatus, Takeuchi, and Eriotremex formosanus, Matsumura, are mentioned as two species originally described from Formosa but shown also to occur on the mainland. Urocerus niger, sp. nov., is described from the Himalayan region, and the name Eriotremex malayanus, sp. nov., is given to a form described without a name by Forsius from Malaya.

9. Several errors in previous work on Siricidae are corrected.

This short review is incomplete in so far as it deals only with the material received by the Imperial Institute of Entomology for identification and that which is contained in the collections of the British Museum. A list of the species unknown to me is given at the end of the paper.

Nine new species and one new variety are described.

i. A four-year survey, involving the investigation of over 1,000 cases of strike, has been made of the species concerned in sheep myiasis throughout the British Isles. In all regions, L. sericata is the principal maggot-fly. In Scotland, Northern England and North Wales, L. caesar is an important species. Phormia terraenovae has much the same distribution, as a striking species, as L. caesar, except that it apparently does not strike sheep in North Wales ; in the areas where it occurs it follows L. caesar in importance. These two species are apparently capable of acting as primary flies. Calliphora erythrocephala, and, more rarely, Calliphora vomitoria occur fairly generally over the country, but only in very low incidence. Larvae of Muscina spp. have been recovered from only three cases, one each in Scotland, England and Ireland. The two closely related species, L. caesar and L. illustris have only been separated in a few instances, and there is not sufficient evidence for the latter species, but it would seem to be more common in lowland areas than in the north and west.

ii. The incidence of “ alternative ” species, i.e. species other than L. sericata, decreases from north-west to south-east of Britain ; they occurred in 47 per cent. of strikes north and west of a line from Inverness to Glasgow, and in over 20 per cent. of the cases between this and a line from East Lothian to Morecambe Bay, south of the Lake District. In the fringe of hill country east and south of this, e.g., the Cheviots, Pennines and Wales, about 10 per cent. of cases contained “ alternative ” species. Farther to the south-east and south “ alternatives ” were rare.

iii. Within the above belts of territory, the “ alternative ” species appear to have certain common regions which are favourable for the striking habit, although the limits of strike-distribution for each species are apparently not quite coincident. These “ favourable regions ” are discussed in the text. With Phormia terraenovae the habit appears to be restricted to favourable regions, where it occurs in moderately high incidence ; with L. caesar strike occurs in relatively high incidence in favourable regions, but is also present, in low proportion, throughout the country, while the Calliphora species strike in low proportion in favourable regions, and sporadically elsewhere.

iv. Phormia and L. caesar have a higher relative incidence in the first half of the fly season than in the later half ; C. vomitoria may be restricted to the extremes of the season-May, June and September.