A basic question? Female / male ratio

[monasteria]

In the butterfly world, females are much rarer than males. That becomes not only obvious when you want to complete a pair for your collection. I would like to know the reason for that ratio, which is a lot different to the one from other species. A female / male ratio of 1 / 40 for example is nothing special I guess. 

Can anyone give answers and solve that issue?

[timmsyrj]

I should think that the sexes occur in equal numbers, but the females lead a more secretive life to avoid predators as they are carrying a valuable batch of eggs, 1 female to 40 male ratio does not seem to make sense with regards to species survival, geographical expansion.

[monasteria]

That makes sense regarding to biological aspects of Species. The 1:40 ratio I mentioned is from a scientific book, but may be related to the number of specimens caught (not the real number of existing specimens). 

[bobw]

The ratio of all species is generally about 1:1 in nature; this becomes apparent when you rear anything in reasonable numbers. As Rich says, the reason males appear to be much commoner in the field, and in the marketplace, is because they're much more visible. They're continually flying around hunting for females whereas the females don't fly as much but tend to stay close to the foodplant depositing their eggs. Males generally emerge earlier than females, often by about a week; early in the flight period you will generally only see males, later in the flight period you'll see more females as the males drop off.  Because of their different behaviour, it's often easy to tell males from females in the field purely by the way they fly.

With many rainforest butterflies, males will come down to ground level to feed whereas females stay up in the treetops where the foodplants are. In some Amazonian species the males only fetch about $1 but females are often worth hundreds. In fact in some species where males are considered common, the females are unknown or have only recently been described.

Bob

[nomad]

It is certainly true of Delias butterflies that the females are much harder to obtain. Most of the specimens for sale are males, the females seem to be rarely captured and females of the rare species seem impossible to obtain and when they are listed, go for much higher prices.

[Adam Cotton]

I must agree with Bobw, breeding generally produces a 1:1 male:female ratio, sometimes even slightly more females than males.

Commercial collecting of butterflies generally involves catching males at river banks, wet mud etc where they gather in large numbers. Females almost never come to wet sand/mud, although I have occasionally caught the odd female (usually either very fresh or very old) at wet sand, but they are always solitary, and are never attracted to the baits that males congregate at in large numbers (urine, rotting crabs etc).

Female butterflies can often be collected at flowers, usually either early in the morning or late afternoon, although many tropical species also fly down to feed on flowers at midday if the weather is cooler and cloudy or misty (especially many Troidini like to fly in these conditions). I have noticed that in the catching season for Byasa laos there do tend to be more males flying down to flowers one day, and more females another. Similarly when pupae I have reared are emerging some days more are male and others more female, but over the brood it balances out.

Adam.