384
THE EFFECTS OF TEMPERATURE AND HUMIDITY
ON THE CHEESE SKIPPER, PIOPHILA CASEI (L.)
BY
JOHN SMART, PH.D., Carnegie Research Scholar.
(Department of Entomology, London School of Hygiene and Tropical Medicine.)
(Received 15th May, 1935.)
(With One Text-figure.)
THE extraordinary resistance of all the developmental stages of Piophila casei (L.)
to adverse conditions and their general tenacity to life has been noticed by several
authors. Alessandrini (1909) reports the results of the treatment of the larvae with
some seventy reagents; Krausse (1909) reports on a similar series of tests, and
Simmons (1927) gives details of some rather more carefully controlled experiments
together with a general summary of our knowledge of the fly at the time of the
publication of his paper. The resistance of the fly in its various stages to conditions
usually lethal for insects, suggested that it might prove of interest to examine some
of the reactions of the Cheese Skipper to conditions of controlled temperature and
humidity such as Buxton (1931 b) and Mellanby (1932,1934) have applied to various
other insects. The present paper is the result of this investigation.
TECHNIQUE.
The larvae, pupae and adults used for the experiments were reared on common
" cheddar" cheese. The cheese was cut up into lumps of about \ in. cube, and
1-pint milk bottles filled to about one-third with these lumps. The bottles were
exposed to ovipositing females for 24-48 hours in a cage and then removed and
stoppered with a plug of cotton-wool. The eggs hatched in a day or two and the
larvae developed rapidly. Within a fortnight or less, according to the temperature,
they were fully developed and began to migrate out of the cheese, which was now
reduced to a soft mass, to find a drier situation for pupation. The migrating larvae
came to rest in the cotton-wool plug into which they insinuated themselves and,
if left there, they pupated.
If the cotton-wool plug was removed daily it might be assumed, for the purposes
of the experiment, that all the larvae obtained daily from a given bottle were in a
similar physiological condition. Pupae and imagines were obtained without any
difficulty by keeping the migrating larvae in containers till they pupated.
For rough exposures to constant temperatures without controlled humidity the
insects were placed in an incubator in a suitable container. When the humidity had
to be controlled and the exposures were 24 hours in length or longer, the "fruit-jar "
The Effects of Temperature and Humidity on the Cheese Skipper
385
method of Buxton and Mellanby (1934) was used. For shorter exposures the
apparatus shown in Fig. 1 was used. This consisted of a large wide-mouthed bottle
(L) of about 1 \ litres capacity filled with water and into
which a cork (Cx) was fitted. This cork (C\) was bored to
receive a 6-in. boiling tube (B) which passed down into
the water in the large bottle. The cork (C\) had to be fitted
very tightly so as to grip the boiling tube and hold it down
into the water; it had, however, a small vent (V) to allow
of changes in the volume of the water and air in the
bottle being compensated. At the bottom of the boiling
tube was a short wide specimen tube (S), containing the humidity controlling mixture, with a suitable
wire attachment to allow of its easy removal when desired.
The cork of the boiling tube (C8) was bored to receive a
short thermometer (T) on the stem of which was a second
tightly fitting cork (C3). The actual container (G) for the
specimens that were being used for an experiment was
made of brass gauze, and it was held in the position
shown by pushing its open end on to the cork (C3) on
the thermometer.
For use the apparatus, with a suitable H2SO4 solution
in the specimen tube (S) to control the humidity, was
placed in a constant temperature water-bath. When the
thermometer (71) indicated that the desired constant tern- „.
„.
.
. ,
v
'
Fig. 1. Diagrammatic section of
perature was being maintained in the gauze container the apparatus used to expose the
the cork (C2) carrying with it the thermometer and the l*™e for PeTioda of ' hour gauze container was removed from the boiling tube, the
mouth of which was quickly covered by an unbored cork. The insects were placed in
the gauze container which was then as rapidly as possible replaced in the boiling tube
along with the thermometer. In actual practice it was found that, within 5 min. from
the time of replacing the gauze container in the boiling tube, the temperature therein
had returned to the constant level at which the apparatus had been running. In all the
experiments the humidity was controlled with H2SO4 solutions prepared according
to the directions of Buxton (1931a).
GENERAL.
The eggs of P. casei hatched and the resultant larvae developed into apparently
normal adults at temperatures up to 350 C. At and above this temperature a
complication is introduced, since the fat contained in the cheese melted and many
larvae were apparently drowned in it. Controlled humidity was of course unobtainable in the containers in which the larvae were bred owing to the nature of the
cheese.
386
JOHN SMART
EXPERIMENTS ON THE LARVAE.
The thermal death-point of the larvae was determined for i- and 24-hour
exposures at relative humidities of o, 30, 60, 90, and 100 per cent.; in all cases the
larvae employed were mature specimens that had migrated out of the cheese into
the cotton-wool plugs of the rearing bottles. The results of the 24-hour exposures
are shown in Table I. Counts for the number of deaths, in each experimental group
of 25-30 larvae, were made after allowing a recovery period of 12-24 hours at 25" C ,
a temperature which appeared to be very nearly optimal for the species.
Table I. Thermal death-point of mature "migrating" larvae o/Piophila casei (L.)
exposed for a period of 24 hours. Thefiguresrepresent the percentage of larvae
killed, in each experimental batch of 25-30 larvae.
Relative humidity %
Temperature ° C.
48
46
45
°
3°
60
90
100
100
100
100
100
95
100
100
40
25
0
100
43
0
0
100
100
100
0
50
1
0
The lowest temperature at which deaths occurred for a 24-hour exposure was
450 C. at a relative humidity of o per cent. The dead larvae were very shrunken and
had undergone obvious " drying "; the survivors were also very shrunken and would
probably have died if the exposure had been prolonged. At this temperature, 450 C ,
there were no deaths at the other humidities, but the larvae exposed to a relative
humidity of 100 per cent, appeared to be very "sick" and their recovery was very
slow. No larvae survived for 24 hours at any relative humidity when subjected to
a constant temperature of 480 C. The relationships of the effects of relative humidity
at the other temperatures between 45 and 480 C. can be seen from the table which
shows that the optimum relative humidity for the survival of P. casei larvae at high
temperatures is about 60 per cent.
The thermal death-point of the larvae for i-hour exposures was found to be
52J0 C , regardless of the relative humidity to which the larvae were subjected. The
larvae survived for an hour at a temperature of 520 C.
These results correspond with those obtained by Mellanby (1934) in experiments
dealing with a number of insects. It may, however, be noted that the temperatures
required to kill the larvae of P. casei are about io° C. higher than those fatal to the
insects used by Mellanby.
EXPERIMENTS ON THE PUPAE.
Pupal life lasted 8 days at a constant temperature of 250 C , and this time was
independent of the relative humidity to which the pupae were subjected. At 300 C.
the time occupied was 6 days, and the same period was required by pupae subjected
The Effects of Temperature and Humidity on the Cheese Skipper
387
0
to a temperature of 35 C. Owing to the complexity of the physiological changes
during the pupal stage and the difficulty of determining the death of a pupa the
experiments carried out involved the exposure of pupae to given constant temperatures and humidities for the whole of their pupal life. It was found that,
regardless of the humidity, at 350 C. 100 per cent., at 360 C. 50 per cent, and at 370 C.
0 per cent., flies emerged from the pupae after making allowance for the natural
death-rate among pupae which, at lower temperatures, appeared to be between
1 and 3 per cent.
This effect was not solely due to temperature. Examination of the individual
puparia showed that in all cases, after allowing for the natural death-rate, the puparia
contained fully metamorphosed flies which, if not actually alive, had died as a
consequence of their having failed to effect an emergence from their puparia. At
the highest temperature (370 C.) approximately 10 per cent, of theflieshad managed
to push open the lid of the puparia but emergence had not been subsequently
effected. At 360 C , as noted above, about 50 per cent, of the flies emerged, the
remainder failing to do so. In nearly all cases these failures succeeded in displacing
the puparial lid, and their heads and in some cases the anterior part of their thoraces
protruded from the opening of the puparium. Often the flies could be seen making
violent but ineffectual efforts to protrude themselves further from the puparium.
During their struggles their ptylinal sacs were expanded to an enormous degree,
and continued to be expanded and contracted long after it was certain that the fly
was not going to effect its escape. Eventually such flies died in their puparia with
the ptylinum usually incompletely withdrawn.
The exact cause of this failure to emerge was not determined. There were some
indications that the proportion of weight lost by the pupae over the whole pupal
period was less at the higher temperatures than at the low, leaving the imagines
formed at the higher temperatures bulkier than they would have been had the
temperature been nearer that at which development usually took place, this extra
bulk being possibly sufficient to prevent their successful emergence from the
puparia.
SUMMARY.
The paper gives the results of a short series of experiments carried out to determine the thermal death-point under conditions of controlled humidity of the larva
and pupa of the Cheese Skipper, Piophila casei (L.). The larva is remarkable for the
high temperatures it can withstand, namely 520 C , for 1 hour's exposure and 450 C.
for an exposure of 24 hours. The death of the pupa at a much lower temperature is
shown to be due to a secondary effect of temperature on its physiology.
ACKNOWLEDGMENTS.
The work on which this paper is based was carried out in the Entomology
Department of the London School of Hygiene and Tropical Medicine during part
of my tenure of a Carnegie Research Scholarship. I am indebted to Prof. P. A.
Buxton for his numerous suggestions and kindly criticism of the work as it proceeded.
388
JOHN
SMART
REFERENCES.
ALESSANDRINI, G. (1909). Arch. Paraiit., Paris, 13, 337-82.
BUXTON, P. A. (1931a). Bull. ent. Res. 22, 431-47.
(1931ft). J. exp. Bid. 8, 275-8.
BUXTON, P. A. and MELLANBY, K. (1934). Bull. ent. Res. 25, 171-5.
KRAUSSE, A. H. (1909). Z. wiu. InsektBiol. 5, 394-8.
MELLANBY, K. (1932). Parantology, 24, 419-28.
(1932)- J- txp- Biol. 9, 222-31.
SIMMONS, P. (1927). Bull. U.S. Dep. Agric, No. 1453.