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Sex Differentiation in Insects. A Few Tips.

Knowing the sex of your bug can be enormously satisfying. Fortunately there are some families and whole groups of related families of insects in which sex determination is relatively easy. I am always amused when I tell somebody that a Syrphid fly is a boy or a girl and they are very impressed and assume that I am a full-fledged entomologist, which I am not. You can give the same impression with just a little knowledge.
But in many other instances it is not that easy. The differences are very minor or not obvious to the beginner, or they apply to only one species or a small group of related species. I will not try to list those here, but if you are curious about a particular bug, you can check the information page and also look at the images in which sex has been determined. For instance look up: Monarch butterflies and Zabulon skippers.
If you want to know about sexual differences in spiders go to: Spiders Information page.
In the immense majority of cases you won’t detect sexual differences in immature insects. Grubs, maggots and caterpillars are not interested in sex, only on eating and growing, so they don’t need any external sexual features.
So, here are a few tips on how to sex insects. I hope you find them helpful.
A very crude rule of thumb is that females tend to be bigger and to have a bulgier abdomen, especially when they are full of eggs. Of course you need to see images of males and females of the same species side by side to be sure.

Cluster flies, Pollenia sp., mating
Male and female

Praying mantis, Mantis religiosa
Female and male

Eyes: The males of several families of flies (Syrphidae, Bombyliidae, Tabanidae, Empididae, Stratiomyidae) have larger eyes than the females. They need that superior vision to locate the girls: “The better to see you with, my dear.” In some cases the male’s eyes are so big that they meet at the top of the head, You will have no trouble identifying the sex of Syrphid flies, for instance.

Syrphid flies, Allograpta obliqua
Female            Male

Bombyliidae, Sparnopolius confusus
Female            Male

Bibionidae
Female and male

Antennae: In many moths the males have feathery antennae and the females have simpler ones. They use their antennae to sniff the aroma, sex pheromones, emitted by the girls. “The better to smell you with, my dear.” They are so good at it that some can find a female of their own species that is more than a mile away.

Luna moths, Actias luna
Female            Male

Flannel moths, Megalopyge opercularis
Female            Male

The males of many species of beetles also have fancier antennae than the females and for the same reason as moths. Some have longer antennae, others have funny looking ones: feathery, comb-like, etc.

Northeastern Pine Sawyer, Monochamus notatus
Male and female.

Wedge-shaped Beetle, Macrosiagon limbata
Female                 Male

Ten lined June Beetle, Polyphylla decemlineata
Female            Male

Many male mosquitoes and midges also have feathery antennae. But in this case their function is very different; they detect vibrations with them: “The better to hear you with, my dear.” The girls produce a sound such as the annoying whine made by bloodthirsty female mosquitoes. That unpleasant hum that keeps us awake at night is music to the ears, I mean antennae, of boy mosquitoes.

Golden saltmarsh mosquitoes, Ochlerotatus sollicitans
Female            Male

Midges, Chironomidae
Female and Male

Male wasps of the Vespidae family have antennae that curve at the tip, while female’s antennae are straight.

Northern Paper wasps, Polistes fuscatus
Female            Male

Pollen baskets or brushes: Among bees, only the females gather pollen to feed the babies and have the equipment to do so. Pollen baskets (corbicula) are on the hind legs. In most cases the brushes (scopa) are on the hind legs, but in leaf-cutter bees they are on the belly.

Halictid bees, Halictus ligatus. (Hairy hind legs in the female).
Female            Male

Megachilid bees (Leaf-cutting bees, mason bees, etc.) (Hairy belly in the female). Megachile
Female            Male

A word of caution: Some bees, such as some Colletidae, don’t have pollen baskets because they carry pollen in their crops. And cuckoo bees don’t carry pollen at all. So the absence of pollen baskets is of no help in such cases.

Megachilid bees, Coelioxys. They parasitize other members of the leaf-cutting family and don’t need pollen baskets.
Female            Male

Cuckoo bees, Nomada. They parasitize other bees and don’t need pollen baskets.
Female            Male

Egg laying organ or Ovipositor: Among Ichneumons and Braconids, in many cases the females have long ovipositors. But another word of caution: the absence of ovipositors doesn’t necessarily mean that you have a male. The ovipositor is very small in some species and the insect carries it folded under the abdomen, so you may not see it.

Giant ichneumons, Megarhyssa macrurus
Female            Male

Female katydids and crickets usually have a very visible ovipositor. Did I mention the word caution before? Use it here too.

Handsome meadow katydid, Orchelimum pulchellum
Female            Male

Weapons In several families of scarab beetles the males compete for sex and carry formidable weapons, reminiscent of the ones of their mammalian counterparts. Not surprisingly they have names such as stag beetles, rhinoceros beetles, etc. Females lack such equipment.

Stag beetles, Lucanus capreolus
Female and Male

Rhinoceros beetles, Dynastes tityus
Female            Male

Behavior can be very informative, only female bees and wasps build nests, so if you see one doing so, you know it is a female. Also, only female wasps carry prey to their nests to feed their babies. The guys don’t concern themselves with chores of housekeeping and childcare.

Cerceris
Female

Gorytes
Female

Glenostictia
Female

And, of course, with any kind of insect, if you see one laying eggs, it is definitely a female.

Please, those that know more than me add any comments, corrections or additions that you can think of. Thanks in advance.

Dolichopodidae
Male secondary sexual characters (MSSC) are found in many Dolichopodidae and often allow sex and genus or species identification from photos without looking at microscopic hairs and other impossible things requested by authors of keys.

This looks like a typical green Sciapodinae, a slightly thinner version of the kind you find on leaves:

Look more closely at the wing, especially the shape of the tip. Wing deformation is a MSSC in many Sciapodinae.

This Dolichopus wears fuzzy slippers on his front feet:



See also: http://www.fcla.edu/FlaEnt/fe80p142.pdf

This is the calos tarsus of Calotarsa (Platypezidae):

consider adding those...
Peculiar antennae in males
in Meloe the male uses his distorted clam-like middle antennomeres to grip his mate's antennae

Collops (no idea how it works)
Longer male antennae in some longhorns
(note also the ovipositor)

Variously dimorphic antennae in many other families...

Males have recessed, or otherwise modified elytral apices in some Melyridae
and in Pedilus

Dilated male protarsi in...
...most ground beetles


...Dysticinae [suction pad]
compare ♀
(Note: also dimorphic in other dytiscid subfamilies, but in a less dramatc fashion)
...and Hydrophilus


Canaliculate female elytra in some Dysticinae


Longer female snout in some weevils


Disproportionate forelimbs in some weevil and leaf beetle males
♂♀

Hirsute forehead in some bark beetle males...
Vermiform females in many families of the Elateroidea...
&c, &c, &c....

 
Nice assortment
Worthy of an article of its own, perhaps. I did mention the fancier or longer antennae of some males, but in general I was trying to keep it very simple. I omitted other things too, such as the glandular black spots of male monarchs.
Many of these images weren't in the guide when I put this article together. Anyway you did such nice work that people can see it here if they want to learn a little more. Thanks.
I love the Meloe antennae, kinky caresses, I guess.

 
Meloe antennae
it's all about control, i'm afraid; cuffing as a matter of routine, not kinkiness :(
i like the malachiids' preposterous gadgetry better.

speaking of blister beetles, though, it's a shame there's no Cerocoma in our area...

Thanks, and about centepedes?
Thanks, great article.
Does anyone know how to identify the sex of a centipede, specifically scolopendra?

 
Not easy
Try reading this.

Some Mirids have different wing structures or lengths.
For example, the male of Orthocephalus species have complete wings including cuneus and membrane, while the female lacks cuneus and membrane.

Male vs Female

 
Sexing Bombyliidae
Under pressure from some others, I am offering an appendix to the sexing of certain Bombyliids using the eye separation. Some Anthracine bee flies, such as Aphoebantus, can be sexed using the eye spacing rule which holds for most non-Anthracine Bombyliidae (such as your Sparnoploius example), but most Anthracinae don't show an obvious sexual dimorphism in this regard. Unfortunately, the Anthracinae constitute the bulk of Bombyliid species, at least the larger ones most likely to be photographed. For some, I have nothing better than turning them around and peering at the naughty bits.

For those interested in butterfly sex differences
David Ferguson wrote a comprehensive descrition of some m/f differences he looks for, in response to a question of mine on this image:


A Painful Difference
You might mention that any wasp, bee or ant that stings is a female, and that only females of most (all?) blood-sucking diptera actually bite.

One caveat is that males of a few wasps have a phony stinger that may be able to penetrate the skin, but has no venom.

Also, I believe that a scopa under the abdomen is found in all the non-parasitic Megachilids, which include more than just leafcutters (genus Megacheile).

Another difference:
In species or groups with winged and wingless forms, the wingless ones tend to be female. A few examples that come to mind: webspinners, velvet ants and bagworm moths. The caveat here is that there are exceptions, so one would have to look it up to confirm.

Dragonflies and flies
According to the answers to my question here and here the tip of the abdomen of a crane fly indicates sex. The same is true of dragonflies, where the male's equipment is used to grab the female by the neck.

The abdomens of flower flies of genus Toxomerus have distinct tips in male and female, but without grasping apparatus. (Is this true of Syrphidae in general?)

Simply outstanding
Beatriz,

This is an invaluable addition to BugGuide!

Wow, I've just learned a lot.

To put it simply, males have more noticeable characteristics than the females, just like in the animal world - male birds are more colorful, male lions have those great manes, etc.

Thanks!!! This should be listed somewhere on the main page or something.

 
Insects are animals too! :)
And there are plenty of exceptions to males being more "noticeable" -female raptors and many female insects and spiders are a lot bigger than their male counterparts, for example...

 
Ah yes, I forgot about insect
Ah yes, I forgot about insect size. How true!

Thanks for this nice article - extremely helpful.
I had a question and a comment:

Re: the use of eye size to sex flies - do you think your list of the diptera families where this is possible is comprehensive? In other words, since Calliphoridae is not on your list, then eye size is not useful in sexing those flies? Any good references for how you came up with this list?

And a minor quibble: "Among bees, only the females gather pollen to feed their babies" - only "their" babies in a very loose sense - they are gathering pollen for the queen's offspring, right? A minor point, but it will help newbies understand the social biology of bees more accurately, I think...

 
Good points, thanks
No, the list is not comprehensive, that is why the subtitle is: “A Few Tips”. I just gave a few examples; there probably are other flies to which the eye size difference applies. I am sure that the same thing goes for antennae and a few other things. I just didn’t want to make the article too long.
I should rephrase the bee part. Solitary bees feed their own babies, but, as you say, the social ones are feeding their sisters. Just substituting the for their would do the trick.

 
I think
that if you want to, you can safely add Bibionidae (march flies) to the list of flies with holoptic, large-eyed males.

From one of several web sites:

"Bibionidae Fleming, 1821

Small to large (3.0-15.0 mm) nematocerous flies with a strong sexual dimorphism that is evident in both the morphology (eyes holoptic in males, broadly separated in females) and the colour..."

Jean-Paul Haenni1 & Jaroslav Bosák2

1Muséum d’histoire naturelle, Rue des Terreaux 14, CH-2000 Neuchâtel, Switzerland; jean-paul.haenni@unine.ch

 
Thanks
And what a difference it is! I included it in the article.

Wonderful Article
I have bookmarked this page along with the Spider Eyes article. Very helpful for a newbie like me. One male-female id that I have had trouble with for some strange reason is Grasshoppers. I know the male is more rounded at the back end but some pictures I have trouble determining. I go through the pictures of grasshoppers and try to guess but not all are labelled male/female.

Thank you so much for taking the time and effort to do this article. I Love it.

Yes, very good, one addition.
Congrats on a great article! One addition I might make is the modification of tarsi in some beetles. Males diving beetles, and some ground beetles, at least, have dilated (expanded) front tarsi. This is most dramatic in dytiscids and Calosoma and Carabus ground beetles. Also, under "antennae," might include examples of male and female Polyphaga, male and female Meloe, and various male and female cerambycids. Lastly, you might address body size with some examples from Orthoptera and Hemiptera (Reduviidae like wheel bugs and Zelus for example). Oh, and some male Coreidae have greatly swollen hind femora, and/or 'kinked' hind tibiae. I'll keep my thinking cap on for this topic:-)

 
Thanks
Thanks for the comments and suggestions. I will try to include the beetles etc. Now, I remember that many male moths have hair pencils or tufts.
Some day I would like to tackle the "adult" vs. "immature" subject in an article. It is amazing how many people leave that field blank because they have no knowledge of complete metamorphosis.

Excellent article!
I was interested to learn that it's possible to sex pupae of at least some leps. I've seen it done for sphingids, I think, though I haven't got a reference, and we have an example for monarchs, here.

Nice!
This is the sort of info I, personally, would like to see more of. Very informative. Thanks for taking the time.

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