The Meadow Brown Sex Ratios

THE MEADOW BROWN ( M A N I O L A JURTINA, L.) SEX RATIOS by

S.

BEAUFOY

THE Meadow Brown is widespread all over the country, and particularly favours rough meadow land, where there are flowers upon which it can feed. T h e insect has only one generation in the year, spending the winter as a half-grown larva. T h e larvae feed upon many types of grass, mainly at night, the day being spent low down in the grass. Emergence starts towards the end of June, the exact time depending upon the weather, and continues throughout July and August. T h e numbers to be seen fall rapidly at the beginning of September. In 1958 a large n u m b e r of the butterflies were caught in a rough meadow just outside a wood in the parish of Bentley, near Ipswich. T h e main object in collecting was in connection with research work on evolution, being carried out by Dr. E. B. Ford, F.R.S., of Oxford University, but the following figures, showing how the ratio of the sexes varies with time, may be of interest.

Date

No. of insects Percentage Male Female Male Female 30/6/58 to 9 / 7 / 5 8 222 14 94 6 10/7/58 to 1 9 / 7 / 5 8 222 71 76 24 21 /7 /58 to 3017 /58 162 124 57 43 31/7/58 to 2 0 / 8 / 5 8 no collecting.

21/8/58 34 116 23 77 3 / 9 / 5 8 and 4 / 9 / 5 8 2 33 6 94 It will thus be seen that, in general, males emerge before the females, so ensuring that there will always be males available for pairing with the females as the latter emerge. Should females emerge first, some of these would die before pairing, so that the species as a whole would suffer.

It would be interesting to determine the relative numbers of the sexes in other species, and suggestions for this are made in the Schools Supplement, Part X I I I to Transactions Vol. X I , Part I I .

Similar counts were made in 1959 in the same area with the following results :— Date 13/6/59 to 2 8 / 6 / 5 9 2 / 7 / 5 9 to 8 / 7 / 5 9 9 / 7 / 5 9 to 1 8 / 7 / 5 9 19/7/59 to 2 8 / 7 / 5 9 29/7/59 to 1 7 / 8 / 5 9 18/7/59

No. of insects Male Female 174 8 268 62 107 118 68 140 no collecting.

0 5

Percentage Male Female 96 4 81 19 48 52 33 67 0

100

252

THE MEADOW

BROWN

It will be seen that the general pattern is the same as in 1958 in that there is a large excess of males over females at the beginning of the season, the reverse being the case at the end of the summer.

The great difference between the relative numbers collected in the two years lies in the time of emergence. In 1958, which was wet and cold, the first butterflies did not appear in any numbers until the end of June, while in the hot and dry summer of 1959 there were considerable numbers Aying as early as the middle of June, and at the end of June and beginning of July they were very plentiful indeed. In 1958, during the period 21st to 30th July, the numbers of males was still in excess of the number of females, while in 1959 the number of males feil below the number of females during the period 9th to 18th July. At the end of August, 1958, the insects were still quite plentiful, while in 1959 the Aying season was over very early, only 5 butterAies being captured on 18th August. This difference between the two years was, no doubt due to the very different weather conditions ; the hot spring of 1959 caused the butterAies to start emerging about a fortnight earlier than in 1958, and the continued sunny weather enabled the insects to be more continuously active than they were during the previous summer. Their length of life in 1959, therefore, was probably shorter, and this would result in a dearth of the insects in the late summer.

TWO VISITS TO SUFFOLK DURING 1959 by

BARON DE W O R M S

SUFFOLK indeed had its fair share of the recent phenomenal summer and collectors who visited that part of England reaped a very good harvest of the lepidoptera of those regions. Once more I joined Mr. Edgar Hare on July 14th when as before we made our headquarters at Southwold. That night we put up our m /v generator on the bank overlooking the extensive marshes between Walberswick and Dunwich, but conditions were far from favourable and we only saw 16 species of moths at our light, none of which was of any special note. The next day proved as usual very bright and warm when we explored the marshy area further. The chief