540 TRANSACTIONS OF SECTION D. 
FRIDAY, AUGUST 2. 
The following Papers and Reports were read :— 
1. Hxpeviments on Seasonally Dimorphic Forms of African Lepidoptera. 
By ¥. A. Dixry, W.A., .D. 
It is now well known that the phenomenon of seasonal dimorphism is of 
common occurrence in tropical and subtropical lepidoptera. In those species that 
produce several broods in the course of the year it is often found that the successive 
broods differ widely in appearance according to the meteorological conditions 
prevailing during their immature stages. The contrast between such seasonal 
phases becomes especially marked in the case of forms inhabiting regions where 
there is a sharp distinction between the periods of rain and of dry weather. 
The succession of these distinct phases is not determined by a regular principle 
of alternation ; for in the instance of quickly breeding forms a series of successive 
generations may follow one another during the same ‘ wet’ or ‘dry’ season, all 
the generations being of the same type, and varying in number according to the 
duration of the ‘ wet’ or ‘dry’ conditions in any particular year. But no sooner 
do the meteorological conditions change than the next emerging brood of the 
same species shows a corresponding alteration, often sudden, but sometimes so 
gradual that it appears to take two or more transitional stages to bring the insect 
up to the full development of the new seasonal phase. This latter phenomenon 
is especially well marked in the genus Bydlia. 
Experiments have been tried with a view to ascertaining the particular stimu- 
lus or combination of stimuli which causes the butterfly to assume its special 
seasonal form at the appropriate time. In the classical instance of the European 
Araschnia prorsa-levana it was found by Dorfmeister, Weismann, Merrifield, and 
others that pup which left to themselves would have produced the ‘summer’ 
form prorsa, will if refrigerated give rise to a phase more or less closely resembling 
the ‘spring’ form levana. 
Similar trials have been made with tropical and subtropical species, but until 
recently with somewhat inconclusive results. Mr. G. A. K. Marshall, however, 
working at Salisbury in Mashonaland, has now succeeded in showing that by 
artificially varying the conditions to which the butterflies are exposed in their 
immature stages it is possible to bring about in the midst of one season the 
emergence of a form which under natural conditions would only have been 
produced in the other. 
Mr. Marshall has further demonstrated by experiment that the period of 
growth during which the butterfly is susceptible to climatic influences varies in 
different species, the critical stage being in some cases confined to the larval, in 
others to the pupal condition. He has also shown in one instance (that of 
Belenois severina) that the effect of moisture combined with heat differs entirely 
from the effect of the former factor alone. 
Some of the actual results of the experiments referred to are exhibited, the 
specimens shown belonging to the genera TYeracolus (T. achine Cram. and 
T. omphale Godt.) and Belenois (B. severina Cram.). The exhibit also includes 
specimens of other groups to illustrate the general principle. 
2. Report on the Occupation of a Table at the Marine Laboratory, 
Plymouth.—See Reports, p. 346. 
3. Functions of the Spiracle in Sharks and Rays. 
By A. D. DarRBisHIRE. 
