Nocturnal forays, part 1: Mellanby

Despite its disobliging disposition, the bed bug must eventually give up its secrets to researchers.

This process is slow and apparently fraught with failure, but what is being learned is there for us to read and understand if only we can get past the language and shorthand that are not meant for us — and in most cases there is in fact no other way to access this information. This is not a simple matter and is, for me at least, a constant struggle.

The goal of today’s exercise is to understand this paper: Romero et al. 2010. Circadian rhythm of spontaneous locomotor activity in the bed bug, Cimex lectularius L. Journal of Insect Physiology doi:10.1016/j.jinsphys.2010.04.025.

But I can’t get there from here, not without a lot of other reading first.

So then, should we begin with the classical study?

Sometime in 1938, Kenneth Mellanby1 discovered an infestation of bed bugs in rat cages at the University of Sheffield. What the rats were being used for he does not say. The infestation had existed for several years.

He did quite a lot with his good fortune, as we’ll see.

The little field work which has previously been carried out on the bed-bug has consisted of examinations of insects which have been discovered and removed from their hiding places. Useful as such results may sometimes be, they give little information about the normal activities of the insects.

I have been able to trap bed-bugs which have left their hiding places voluntarily during their periods of normal activity. Examinations of these insects tell us a good deal about their habits.

Does he sound pleased with himself or what?

Mellanby, Kenneth. 1939. The Physiology and Activity of the Bed-Bug (Cimex Lectularius L.) in a Natural Infestation. Parasitology 31: 200-211. doi:10.1017/S0031182000012762.

There was a lot going on in this rat room. For one thing, there were cockroaches there which he also took the opportunity to study, but Mellanby believed that they were not interested in the bed bugs (and references another study to that effect).

As one would expect, the rats in this room ate some of the bed bugs!2 But not too many of them, or not as many as they would in labs where Mellanby had previously found that “nearly all the bugs are eaten and very few feed and survive.” The difference consisted apparently in the number of rats occupying a cage. When “two rats tended to curl up together and sleep” the bugs were ignored and therefore more successful.

The room was 4 by 3.5 meters and 3.5 meters high. Two meters above the floor there were three windows, but there was no direct sunlight. The rats were in cages on iron racks covering the walls up to the windows. The temperature in the steam-heated room was somewhere between 20-27°C (68-80.6°F).

The floor was of concrete and the walls of smooth plaster, but cracks and particularly the spaces surrounding the pipes in the walls afforded shelter to the insects. Most of the bugs, however, seemed to lurk in the crevices in the metal cages containing the rats, and these animals seemed to form the only food supply for the bugs.

Mellanby collected bed bugs in this room by means of traps placed along the walls and in the middle of the room. I will have a great deal to say about this “Demon” cockroach trap but for now let’s just see what he says of them:

Two kinds of traps have been used, “Demon” cockroach traps and rolls of corrugated paper. In each experiment four “Demon” traps were placed, each one always on the same spot on the floor of the rat room. The bugs in their wanderings climbed up the sides of the traps, fell in and appeared to be unable to climb out again. All stages of nymphs as well as adults were caught by this method. In the second method small rolls of corrugated paper were left on the floor of the room, each roll consisting of a cylinder of paper 10 cm. long and 4 cm. in diameter; the bugs found hiding places in the folds of the paper.

So, yes. What in the world is a Demon cockroach trap and how came it to trap so many bed bugs? I am relieved that after a long, frustrating search I will be able to answer the first part of that question, in good time.

The principal difference in the catches made by the two methods of trapping is that 77.9% of the bugs caught in the “Demon” traps were unfed, whereas only 18.1% of those found in the corrugated paper were unfed. The reason for this is easily seen. Once a bug was inside a “Demon” trap, it was unable to escape, but when a bug crawled inside a roll of corrugated paper it could as easily crawl out again. Hungry bugs seeking food would not remain in the paper, but fed individuals would find a snug hiding place.

This is extremely interesting and here Mellanby makes certain assumptions that we should pay attention to:

It is probable that the “Demon” traps caught such a small proportion of fed bugs because many newly gorged individuals, being less agile after having ingested a considerable weight of blood, tend to remain near the cages in which they have fed. Evidence given later indicates that practically all the unfed bugs captured in the “Demon” traps were hungry individuals which had left their hiding places to seek for food.

I honestly don’t understand the “evidence given later” which seems to hinge on the egg production of females during the first two days after capture (60% of them laid eggs). (My copy of this article is annotated by a previous reader and he or she marked this section just as I would have.)

It was thought at first that some of the females caught in the traps might not have been searching for food but rather for places in which to lay eggs.  Examination of those females which had actually fed before being caught showed this view to be incorrect, for both these bugs and the unfed individuals contained fully-developed eggs in their bodies in the same proportions.

I’m not sure what to make of this and I wish I could talk to my fellow reader (across how many years?) who also marked the above section with an arrow and, I imagine, understood it perfectly.

This, by the way, is the study that allowed Mellanby to calculate the time between feedings (he painted individual bed bugs and recaptured them, a whole interesting subject in itself), at approximately 6 days. [Update 1/8/11: I made an interesting mistake here (and also here). Or, put another way, recaptures may indeed be fascinating, but this is not really how Mellanby derived his conclusions about the time between feedings. I will correct this as soon as I can.]3 For a modern research update on feeding intervals, see this.

Peak catch

On four nights in May 1938, Demon traps were examined every 3 hours each night. Each removal took less than 5 minutes and the light was not turned on. Still, Mellanby reports that during the day the bugs’ behavior was inhibited by workers entering the rat room, but that this did not seem to happen at night, as long as a light was not turned on, based on the numbers of bed bugs caught on nights when no one entered the room.

The maximum catch was obtained at the same time each night. Each point in the graph indicates the number of bugs collected during a period of 3 hr., and the times marked on the abscissa indicate the middle of each period. Thus 0130 represents the collection made between midnight and 3 a.m. It appears that the bugs were most active after 3 a.m. During most of this period the room was not dark, for the sun rose shortly after 4 a.m.

Activity of bed bugs in animal house as measured by the number of individuals captured in traps during 3 hr. periods.  May 1938.  Mellanby (1939) The Physiology and Activity of the Bed-Bug Cimex Lectularius L. in a Natural Infestation

Activity of bed bugs in rat room. May 1938. K. Mellanby (1939) The Physiology and Activity of the Bed-Bug ( Cimex Lectularius L.) in a Natural Infestation.

This means that most bed bugs were caught in Mellanby’s traps sometime between 3 a.m. and 6 a.m. — with the sun rising after 4 (GMT+1, British Summer Time).

Hmmm!

Daytime activity and artificial light

Though in May the bugs were mostly active at, and after, dawn, the presence of a 60 W. electric lamp left burning all night affected the catch considerably. At this time of year the electric light would give greater illumination than daylight in the animal room until about 5:30 a.m., after which daylight would be the brighter.

On two nights when the lamp was left burning, the trap catch was greatly reduced to 24 bed bugs from a high of 65 when no light was left on.

Bed bugs were routinely caught during the day as well as night.

Several catches were made between 9 a.m. and 6 p.m.; on several weekdays no bugs or only one were caught, but on Sundays when the room was not entered all day as many as nineteen bugs were caught. This was during the part of May 1938 when the average nightly catch was sixty-five.

Complete darkness

On one occasion the animal room was kept in complete darkness for a period of 45 hr., from midday on Saturday until Monday morning. The traps were examined throughout the whole period at 3-hourly intervals. The darkness during the day did not in any way upset the rhythm of the bug’s activity, for catches made during the day and night were indistinguishable from those made under natural conditions of daylight and darkness.

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  1. Mellanby is an interesting figure and you can get an idea of the breadth and impact of his work in this obituary. []
  2. Are humans the only chumps who do not go in for Eat or Be Eaten when it comes to bed bugs? I know, but medicinal uses are not quite the same thing! We’re clearly talking bloody, real-time combat here. []
  3. In this article he also delves into his findings of the effect of movement on starved bed bugs (they lose weight at a significantly faster rate after 5 minutes of activity). Previously mentioned here. More Mellanby here (though not a particularly pleasant subject, if you take that as warning). []
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