6. Development, capacity, limitation
Warmth is a basic factor in the development from embryo to reproducing adult. Each species has its specific limitation in the minimal required and the maximal tolerated temperatures. Other environmental factors are involved also. Some experiments with the small heath have shed some light on this.
Butterflies are poikilothermic, that means that their body temperature depends on the temperature of the environment; their activity is limited to between about 10°C and 35°C. Below this temperature the metabolic activity will stop, above it the animal will die.
Butterflies are smart, they warm their bodies by basking in the sunshine to reach a desired body temperature above 30°C. In some species the desired body temperature is about 20°C whereas others tolerate a temperature up to 45°C. In homoeothermic animals the body temperature is kept at 37°C, the ideal temperature for enzyme function but it costs energy to keep the body at this temperature. Poikilothermic animals are in this way economical and they are able to survive because of their very low energy consumption. However, butterflies need a body temperature above 20°C to be able to fly and a shivering butterfly is the well-known phenomenon of one that is too cold to fly.
Table 6-1. Differences in developing time expressed as % of the time per stage.
Table 6-2. Periods of time, as a %, in each stage in the development cycle.
||51% + 26%
||18% + 38%
Growth and warmth
Many experiments are carried out to measure the relationship of larval growth and the environmental temperature. Below the minimal required temperature larval development stops, at a higher temperature the larva starts growing and it grows faster the more the temperature rises till it reaches the point that growth slows and by further warming the insect will die. This is not a linear relationship and it will produce a J-shaped figure by graphic description. This can be described mathematically as an orthogonal hyperbola: X² - Y² = A² in which X stands for temperature and Y for days of development. The symbol A stands for the constant of time (days) and warmth (temperature) and is expressed as degree-days and written as °d, a symbol for the total sum of warmth.
A hyperbola is determined by the limits of required temperature for development and by the lethal temperature. These limits of environmental temperatures are usually about 10°C and 40°C.
Anomalies with the Small Heath
The results of experiments with the small heath (Coenonympha pamphilus) when exposed to an artificial day length of 18 hours and at 15°C and 20°C constant temperatures showed that the larvae grew very slowly and their development from hatching from the egg until pupation took more than 20 weeks. At higher temperatures the development quickens, at 25°C this development finished in 25 days and at 30°C in only 20 days. Thus from 20°C to 25°C rate of development increased from 140 days to 25 days. A description by an orthogonal hyperbola failed completely!
Experiments on butterflies can produce an insight into the capacity and limitations of larval development and also adult functioning.
The small heath (Coenonympha pamphilus) turned out to be a highly evolved species in comparison with the other species of the same genus.
It is able to live in various types of climate and to recover its population from disasters.
Photograph: Jeroen Voogd ©.
No larval diapause occurred in the offspring of butterflies of the small heath captured in the Netherlands (Latitude 52°) that hatched before 12 July because of the long daylight time of more than 16 hours and 13 minutes. However, diapause occurs in all larvae that hatch after 3rd of August when daylight is reduced at that date to 15 hours and 13 minutes. After hatching the larvae will in diapause seven weeks later and overwinter in third instar. Sensitivity to day length should be taken into account when measuring temperature response.
A long period of high humidity can be deleterious for larvae because of the high risk of mould or bacterial attack. In a test, hibernating larvae of the small heath were exposed to different levels of atmospheric humidity during three months of cold weather. At a relative humidity of 60% there was hardly any mortality and the grass on which the larvae hibernated was fully dissected. However, at 100% humidity there was 100% mortality. This may happen in nature in high and thick grass vegetation. At the end of the experiment, the larvae had become black, suggesting they were killed by a disease. So the small heath larva is sensitive to high air humidity.
Ryegrass (Lolium perenne) grows well under both well fertilised and also on rather poor soils.
Relationships of plant quality and larval growth is measured easily. Ryegrass has the reputation of a bad food plant for lepidopteran larvae, however in the experiment ranging from well fertilized to unfertilized soils it turned out that in both situations the larvae of the small heath accept this grass and grow well. Only in the experiment with a maximal dose of fertilizer were some abnormalities in pupal development observed. So it is not the grass which is the problem but agricultural practise.
However, plant species respond differently to soil fertility. An experiment with purple moor grass (Molinia caerulea) demonstrated that this grass follows its own seasonal rhythm. It absorbs easily extra nutrients but in the autumn the quality as a food plant for larvae declines. The species used in the test was the wall (Lasiommata megera), which has not been observed on this plant species in nature. During the experiment this plant turned out to be a very good food-plant for the larvae, but in the autumn the tissue of the blades had a much lower nitrogen content (indication of protein availability) and hence the growing condition of the larvae was poor in the second brood; pupal weight was lower and the induction of hibernation was much higher. The early summer brood of the wall grew well on purple moor grass but not in late summer. Finally it proved to be a poor host-plant for this butterfly species.
Specific heat tolerance
Among the various butterfly species there is a great variety in heat tolerance. A good example is the Arran brown (Erebia ligea) a species of boreal and mountainous regions. In breeding experiments the larvae looked for shade as soon the atmospheric temperature rose to 22°C. On the other hand the larvae of the grayling (Hipparchia semele), a species of temperate and mediterranean climates, did not seek shelter when the temperature rose to 45°C. It is easy to understand where one can expect to find these species in the Benelux: the former in the cool parts of the Ardennes near to the German border and the latter in heathland and drift sands where it can become very hot in summer.
Biological and meteorological data
The question concerning the limitation of the distribution areas of species by climate can be resolved by comparing the length of the period of suitable warmth and the required period for development of the life cycle of the butterflies. The northern limit can be found by comparing the ‘climate window’ and the required time for the overwintering generation. (Further reading chapter 14, ‘Climate matrix’) Species with a fast larval development, such as the orange tip (Anthocharis cardamines) can live in climates with a short season, but also species in which larval development takes more than one year such as the Arran brown. The former requires a climate window of at least 8 weeks, the Arran brown 17 weeks per year to reach the required 34 weeks in two years to complete its development. Knowing this the northern limit can be calculated. However, there is a problem in using data from meteorological stations because there may be local differences due to geomorphology of the landscape. South facing slopes of hills are much warmer than north facing ones, east sides of mountains may be much dryer than their west sides because, if the moisture laden wind emanates from the west, they are situated in the rain shadow.
There are species which always produce a second brood like the pale clouded yellow (Colias hyale). The required warm periods for such a species is calculated by addition of the time of development of the summer brood and that of the overwintering brood, making 7 weeks plus 14 weeks, a total of 21 weeks for the pale clouded yellow.