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Something I was working on in the past involved a species of sentient insectoids. The idea was the two sexes usually live in two very different environments and rarely come into contact with the other one, except for when they meet in a sort of 'middle ground' to mate.

Consequently, they look to outsiders (including humans) like two completely different species (we'll assume the embryos, or whatever equivalent they would have, are left to hatch in the aforementioned neutral ground, and once born those of each sex find their 'home turf' by instinct).

The males of this species would be capable of flight (probably even hovering) with limbs that while highly dexterous are also somewhat fragile. The females on the other hand are flightless, with segmented bodies that are stronger and tougher at the expense of dexterity (any arms are not much more articulate, if at all, than pincers).

In what kind of environment/ climate would these insects develop these very different physical traits? What kind environment would be suitable for them to come into contact long enough to reproduce?

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  • $\begingroup$ I don't know what particular environment would promote sexual dimorphism, but you would need low gravity for beings with an exoskeleton that were big enough to be sentient, particularly if they were capable of flight. $\endgroup$ – Lostinfrance Jul 1 '16 at 8:29
  • $\begingroup$ @Lostinfrance That's not strictly true. Exoskeletons are not by themselves enough to stop larger size or intelligence. $\endgroup$ – Tim B Jul 1 '16 at 13:05
  • $\begingroup$ I've edited the title to try to include the actual question about the aliens. $\endgroup$ – ckersch Jul 6 '16 at 19:56
  • $\begingroup$ Also, as an aside: ability to survive on instinct alone tends to vary inversely with generalized learning ability, which is critical for sentience. It's likely that the adults will care for at least one gender of the young, rather than leaving them to find their own way. $\endgroup$ – ckersch Jul 6 '16 at 19:59
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This strategy is known in some moths, such as the Winter Moth and the Vaporour. The female has a short life as an adult, she doesn't feed but remains by the cocoon. The male finds her by pheromones.

Since the female is vulnerable to predation she is cryptically coloured, and therefore it is important that her pheromones are effective, as the male can't find her by sight. The females don't select their mates so males are not adapted to attract females, only to find them.

The difference from the situation that you describe is that the larvae are mixed, and remain so until the adult emerge from pupae. If the development of the male required a particular food type there could be pressure for the species to separate their genders earlier.

So the environment in which this evolved on Earth is one in which the ancestral species had pheromone signalling, and and there are costly adaptations (wings). The species have a long childhood and short adulthood.

The female benefits by saving in the costs of growing wings. But loses the ability to choose her mate and is somewhat more vulnerable.

How this would affect intelligence is interesting. Are the adults just sex machines or do they engage in rational behaviour. If the latter then the questioning of gender roles becomes interesting. The females instinctive sexual behaviour is to hide and pump pheromone, but a sentient species may question that role, leading to conflict. Most males are controlled by the their pheromone primed sex drive - but what if one actually wants to try communicating to a female.

If the adults are short lived they could lose their brains during pupation. Are then the larvae intellectual. As they are growing, how do they feel about the fact that sometime they will build a cocoon and die soon after. That is some kind of coming-of-age story. In the moths the larvae are not dimorphic. Do the larvae even know their own gender?

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Insects can also be greatly shaped by diet, the presence of certain fruits or plants could lead to very different physical and behavioral traits. You may not even need to drastically separate the species as you could have a tiered rainforest situation where the males occupy the upper canopy/emergent layer and fly above the treeline catching food while the females are down on the forest floor and develop sturdier bodies lacking evasive means.

In such a forest, certain clearings or in the understory (the medium ground between canopy and floor) would be places where they could gather to mate.

There are also species of fungi that "hijack" the behavior of ants and control them to help the fungus reproduce.

If you want to make the species even more confusing to outsiders, the larval stage could be aquatic and they mate in spawning pools. This could even give the appearance of them being three unique species to outsiders when they are actually one.

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Does it have to be the males which fly and the females which are ground bound? Because if you reverse that, then you can have some biological justification.

For instance - the females migrate, the males are territorial.

The females fly off to sea, like albatrosses, and spend the year feeding up on fat-rich fish and squid, so that they can obtain all the right nutrients to lay their clutch of eggs. They'll stop off on islands and coastlines and hang out with other females. The humans may think of them as 'flying Polynesian islanders'.

The mature males are territorial, like male Grevy's zebras or IIRC male Grant's gazelles. (Not all of those males hold territories - some migrate - so they are not an exact match for your insects, but at least they are not migrating by flying). The mature males stay put because come the breeding season, the guys with the territories get all the gals.

If you want them to have 'civilised' territoriality (you get on with your neighbours and invite them round for tea) rather than 'animal' territoriality (you spend time threatening your neighbours and attack anyone who enters you territory) then the males could spend all year building stuff that will convince the females to mate with them. Fantastic pools for the larvae, blinged up houses to show off their wealth (like bower birds), stockpiles of a particular food which will increase clutch size or which is something the larvae eat. The humans could view this as artistic endeavours, religious fervour or some sort of potlatch society.

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