Although traditional weather lore has it that When fleas do very many grow Then it will surely rain or snow, the accepted wisdom nowadays has it the other way around. It seems that fleas and many other insects increase and multiply with great efficiency when temperatures are above normal. And since for some years now we have had mild winters and warm summers, we are potentially susceptible to an explosion in these populations.
Milder conditions, moreover, may change the habitable range of many insects, which some time ago led an expert on these matters, confirming reports that many unusual specimens had been spotted in the south of England, to warn that worse might be to come. "Termites," he declared, in tones reminiscent of the Franco-Prussian War, "have already entered Paris and may soon succeed in breaching British borders." But how does one learn to be an expert? I found an answer to this question in a text-book published three quarters of a century ago which described experiments that were simplicity itself to follow.
For one investigation a transparent container was required, of the largest size that would comfortably fit in a refrigerator, and inside which a small thermometer was placed. The further instructions were as follows: "Introduce a few individuals of any suitably-sized flying insect - house flies, mosquitoes or midges, for example - and place the container in the refrigerator. When it has cooled sufficiently to render the insects immobile, remove it and allow it to warm slowly to room temperature. Note the temperature at which spontaneous flight takes place, and record your observations of the insects' behaviour during the pre-flight period." And you must da capo, as musicians say, with different kinds of insect.
A second exercise is more complex, and investigates the important relationship between "thermal gradients and temperature preferenda". For this a block of metal about 2 inches wide and 2 feet long is needed, together with a purpose-built perspex cover to prevent flying insects from taking wing and disappearing, which the sensible creatures would do at the very earliest of opportunities. A tray of ice is placed on one end of the bar, and a lighted candle under the other, thus establishing a gradient of temperature along the length of metal.
Then: "Put the experimental animals - house flies or meal-worm larvae - into the chamber, scattering them at random along the length of the bar. Leave the apparatus under uniform lighting conditions and plot the distribution of individual animals at regular intervals." What pleasanter pastime could there be to fill an idle hour or two at Christmastide?