I found a beautiful scarce-swallowtail butterfly in my storeroom, but it was kind of frozen and couldn't fly away. So, I put it on my terrace in the sun. After a little bit, it flew a few meters away and fell. I was wondering what could these beings eat, so that I can help it... Any answers would be great appreciated! Thanks in advance
2$\begingroup$ Nectar is one of them. Perhaps some weird species also have some unusual diets... $\endgroup$– JamesApr 25, 2017 at 9:15
1$\begingroup$ @YvesLaFayette It's probably done reproducing, so there's no reason for it to eat or fly any more. Sorry... $\endgroup$– Dmitry GrigoryevApr 25, 2017 at 15:01
1$\begingroup$ Butterflies die. :( That's probably what yours is doing. But if you have a blooming butterfly bush or any tubular flowers in bloom right now, it's worth a shot. $\endgroup$– anongoodnurseApr 25, 2017 at 15:51
$\begingroup$ More probably it's dying (butterflies know how to find food by themselves), or there's a small chance your sugar concentration was too high or too low? $\endgroup$– RodrigoApr 26, 2017 at 16:08
1$\begingroup$ Be careful you don't overfeed them. $\endgroup$– Jason CApr 26, 2017 at 22:15
Adult butterflies don't eat! I mean.... not in the sense of chewing on food. They rather drink. They get their nutrients via ingestion of liquid substances. Their mouth consists of a long tube called a proboscis that acts as a straw.
What do butterflies feed on?
The vast majority of butterflies eat nectar from flowers. Many species are quite specialized and feed on the nectar of a few species only.
There are a few exceptions though. Some species feed on tree sap, dung, mud (see mud-puddling), pollen, or rotting fruit. Butterflies are attracted to sodium and may as well try to feed on human sweat but I doubt there exist any species would get a non-negligible source of nutrients from human sweat. Coming to exceptions, you should definitely consider reading @DmitryGrigoryev's answer as well.
What do caterpillars feed on?
Of course, you are probably aware that butterflies have a complex (-ish) life cycle. The larva is called a caterpillar, while the adult is called a butterfly. So what do caterpillars eat?
Caterpillars have mandibles, so they can chew tissues and not just drink. Most caterpillars eat leaves and other plants parts. Again, many species are specialized to only a few plant species.
There are of course exceptions. For example in the Phengaris genus, (such as the famous large blue (Phengaris arion), caterpillars live in ant colonies and mimic the larvae of these ants. Some of them are able to trick the ants into feeding them as though they were ant larvae (e.g. Phengaris alcon, Phengaris rebeli). Others (e.g. the aforementioned Phengaris arion) will act as predators and eat the real ant larvae!
When the butterflies emerge from their cocoons after metamorphosis, the ants finally figure out the trickery and start attacking the butterfly. The butterfly has therefore very little time to try to fly away from the colony before getting killed. It is a critical life stage for individuals of this species.
How to feed a butterfly?
This section exists thanks to @Rodrigo's recommendation.
As a substitute of nectar one can simply use sugar water. The optimal sugar concentration (in mass-mass) is around 35% (Kim et al. 2011) although it likely varies among species.
Kim et al. 2011 explains that what limits optimal sugar concentration for insects is the viscosity of the solution. Too much sugar renders the solution too viscous and difficult to forage on. Interestingly, the ability to drink viscous solutions depends upon the drinking technic (active suction, capillary suction or viscous dipping) which vary, not among butterflies but among insects. The article is interesting.
$\begingroup$ Then there are other metamorphic species, e.g., Luna Moth, which don't even have a mouth in their 'adult' stage. They don't eat at all after exiting the coccoon. Oh, never mind, Dmitry has that covered. $\endgroup$ Apr 25, 2017 at 18:13
$\begingroup$ It would be great if you specify the limits of sugar concentration butterflies can drink on. I know the amount is different between bees and hummingbirds, but don't remember the numbers, and don't know about butterflies either. $\endgroup$– RodrigoApr 26, 2017 at 16:04
$\begingroup$ OP, do you have a source for "When the butterflies emerge from their cocoons ... the ants finally figure out the trickery and start attacking the butterfly"? The Wikipedia article for one of the Phengaris species cites a book called "Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals" in support of this, but I can't find a peer-reviewed paper or other authoritative source to back it up. Footage from the BBC's "Life on Earth" shows the butterfly leaving the nest unhindered, and there are anecdotal claims that Large Blues can even have the ants escort them outside! $\endgroup$ Dec 14, 2019 at 23:07
1$\begingroup$ @Astrid_Redfern No, sorry I don't have a peer-review reference for it. I am just repeating stories I have heard! $\endgroup$– Remi.bDec 15, 2019 at 21:15
Several species of the order Lepidoptera don't feed at all in adult form, surviving entirely on the reserves made while they were larva. Two examples I'm aware of are the Atlas moth (as well as most of the family it belongs to) and the clothing moth.
Also, many butterflies and moths which normally do feed stop doing so after the mating (for males) or after laying all the eggs (for females). This might be the case of the butterfly you have found: if it has completed its reproductive program, it just looks for a place to pass away in peace and no amount of food and sunlight will revive it.