Biological Clocks in Mosquitoes - Section 1
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Here, a double-plot format (named as the "photoperiodogram" for convenience) is used to show how the percentage of total daily activity varies per consecutive half-hour of the normal solar 24h-day. The format (explained below) shows also how, by overlaying the activity patterns in various light-dark (LD) regimes, the control of the timing of activity can be revealed. The photoperiodograms and commentaries for the fourteen species studied by me (details of which can be found in the Appendices), together with evidence for An. gambiae (drawn from Jones et al., 1972) and Cx. p. molestus (drawn from Chiba & Tomioka, 1992), have been arranged in five groups. On the basis of the flight activity patterns in LD regimes which would be experienced by each species in their known natural geographical ranges, these are -
As many species of mosquito display field activity which is bimodal, most commonly with peaks approximating to dawn and dusk, the terms "early" and "late" crepuscular should be taken as referring to the dusk activity. In all instances, the cited field activity patterns of non-tropical species will have been observed during summer months.
- Fully day-active
- Early (strong light) crepuscular
- Median crepuscular
- Late (very low light) crepuscular
- Fully night-active
The photoperiodograms
The actual records of activity were in the form of event recorder charts. For each mosquito, the trace on a waxed-paper roll was scored on the basis of 1 for activity in any single minute, and transcribed to results sheets as the activity score per half-hour (i.e. maximum score 30). Next, for each half-hour the activity score for the several individuals in the particular experiment was summed and converted to a percentage of the total activity for the calendar day. The mean percentage activity for the duration of the exposure (2-3 days or more) to the particular LD regime was calculated and a line-graph derived of activity against time (0-24h). The merit of using the mean percentage activity is that this enables ready comparison of when activity takes place and minimises the added confusion of variations in the actual amount of activity; which, for instance, can be suppressed or enhanced by light or dark, depending on the species.
The photoperiodograms show a double plot of the 24h-cycle, this is so that the complete cycle may be seen more easily. The line-graph for each LD regime is plotted centred on the mid-day of the 24h-cycle, and along the lateral appropriate to that LD. The photoperiodogram is completed by diagonals representing light-on and light-off, with L = 0h at the base and L = 24h at the top.
Basically similar formats were adopted by Tyschen (1978) and Moore & Rankin (1993), and this form of visual presentation, perhaps, is simpler to appreciate than the photoperiodograms used earlier by Taylor (1969a, b, 1977) and by Chiba & Tomioka (1992). The number of "mosquito-days" (i.e. the number of individuals multiplied by the number of days in which each was recorded) from which each LD plot was calculated is shown below the X-axis.
In interpreting the results, the convention of Pittendrigh & Daan (1976), with additions by Jones & Gubbins (1979), has been followed to designate the observed peaks of activity:-
E = evening, dusk. M = morning, dawn. N = night, midnight.
On a purist note, it should be realised that the way the graphs are plotted by the spreadsheet software means that each activity point appears above an X-axis point which actually is 15 mins ahead of the photoperiod line. This can be seen easily on the NOON and MIDNIGHT verticals, as although they are printed on the line, each activity point represents the level of activity in the half-hour commencing from the time point.
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©1998, 2010 - Brian Taylor CBiol FSB FRES 11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K. Comments to dr.b.taylor@ntlworld.com |