Determining and Annotating the Sex of Hoverflies in iNat

You may find it productive to add the sex annotation to hoverflies - it can be a lot quicker than identifying, and it's a good way for a starter to see the range of hoverfly diversity. It is useful to record the sex of a hoverfly, as they can have different behavioural patterns, they don’t necessarily occur in a 1:1 ratio, and they may also have different phenology. They can also look different, and it’s useful for interested learners, iNat identifiers, or indeed researchers, to be able to call up all the male observations, or all the females. When I (MV) identify Syrphus or Platycheirus for example, I usually identify the males and females separately, because there are different features to look for.
There are two things to address here:
 1) How do you tell the sex of a hoverfly?
 2) What is the best way to add the sex annotation?

Thankfully, determining the sex of a hoverfly is usually easy. Adding the sex of a hoverfly to other people's observations in iNat is also easy, and quick to do in large numbers. (To add the sex to your own observations as you upload see here). Let’s take the second thing first.

Most efficient way of adding annotations
The quickest way to add annotations in iNat is using the ‘Identify’ portal. Once you have filtered for ‘Hoverflies’, ‘United Kingdom’, (and for your own observations if you are only doing your own after upload), click on the first observation you want to add the sex to and navigate to the ‘Annotations’ tab.

This link will get you all the UK Hoverflies without a sex annotation:
If you want to see just your own observations add ‘&user_id=YOUR USERNAME’ to the end of the URL.

Annotations can be added by clicking, but it is quicker to use keyboard shortcuts: for sex use the combination ‘s’ (for sex) followed by ‘m’ (for male), ‘f’ (for female) or ‘c’ (for cannot be determined). Please do not use ‘cannot be determined’ just because you cannot determine it - it means that determination is impossible. Note that annotations do not work like IDs in iNat - if you get it wrong other people cannot out-vote you.

(It is also very helpful to add the annotation for ‘life stage = larva’ which enables the Hoverfly larvae project to collect the observation. Use ‘l’ (for life stage) and ‘l’ again for ‘larva’. Similarly 'l'+'a' for adult and 'l'+'p' for pupa.)

Use the arrow keys to move onto the next observation.

Using this method it is possible to add annotations to a large number of observations very quickly.

Unfortunately there is no way to annotate for a mating pair using the main annotation function. Leave these without a sex annotation.

You may find it easier still if you filter to a smaller taxon - for example Subfamily Syrphinae or a Genus e.g. Eristalis. That way you are looking at the same sort of fly every time making recognition quicker, and it also reduces the number of exceptions you have to think about.

Determining the sex of a hoverfly
It is possible to tell the sex of a hoverfly from behind if you can tell the difference between a male genital capsule and a retracted female ovipostor - but by far the easiest way to tell is using the eyes. In almost all species, the eyes of a male are connected above the antennae (called ‘holoptic’), whereas the females’ eyes are widely separated (‘dichoptic’).

Females Males

In the Subfamily Syrphinae there are no exceptions to this rule and, regardless of the species, absolutely any hoverfly with connected eyes is definitely male (no exceptions there). There are however some groups where the male eyes are separated. The main exceptions are those genera in the Subtribe Helophilina with stripes on top of the thorax (i.e. the genera Helophilus, Parhelophilus, Anasimyia, Eurimyia and Lejops). In these genera the male eyes are closer together, but not by much. In the males however the edges of the eyes are parallel at the top of the head, whereas in the females they diverge; there are often also markings on the female frons that are different to males - this should be checked for individual species. The example below is Helophilus pendulus

Female Male

Other groups that are exceptions are the rarely recorded genus Microdon (the only British genus in the Subfamily Microdontinae: Ant-flies), the subtribe Spheginina (Genera Neoascia and Sphegina) which are delicate small flies with a long narrow waist and a bulbous tip to the abdomen, and the genus Lejogaster, shiny dark metallic flies with a concave face and rather bulbous antennae. These are very difficult to tell the sex, best to leave them unsexed to be honest. Familiarise yourself with how they look using the links to Steven Falk's pictures, and you will be able to steer clear of them.

A very important isolated exception is the common Eristalinus sepulchralis: in this case again the male eyes are parallel to convergent at the top of the head and the females' are divergent, but compared to Helophilus etc there is a much greater difference between the sexes - i.e. the females' eyes are significantly more separated than the males'.

Another isolated exception is the very rare Eristalis abusiva, although in that species the eyes come so close that they are unlikely to cause confusion.

MALE Eristalinus sepulchralis MALE Eristalis abusiva (Photo credit @jeanpaulboerekamps)

Steven Falk’s Flickr is an ideal source for checking photographs of the exceptions mentioned above.

A final note: you will occasionally see a hoverfly with its abdomen greatly extended into a long pointed tube shape: this is the 'ovipostor'. Usually the ovipostor is retracted inside the abdomen so you can't see it. It is extended for the purpose of laying eggs - obviously therefore a hoverfly with an ovipostor is a female!

Posted on 28 June, 2023 13:53 by matthewvosper matthewvosper


Been trying a bit of this over the last couple of days. Should we have a policy for dealing with situations where there are multiple hoverflies of differing gender in a single shot (e.g. mating pairs) or multiple photos including insects of differing gender?

Posted by rkl 8 months ago

@rkl personally I usually just leave them unannotated if the observer hasn't specified one individual as the subject of the observation. I don't think we can annotate that situation in a way that will make sense at the other end. If there's several individuals in the picture but only one can be sexed then sometimes I annotate that.

Two more posts coming up soon on the subject of annotations btw - a bit of a flurry or articles still in these early days :)

Posted by matthewvosper 8 months ago

In the case of multiple photos of differing gender could gentle observer education be the way forward?

Posted by rkl 8 months ago

I suppose it's possible that that might have some effect. I suspect however that annotating them will just lead to some confusion when Roger and Stuart see them. It seems best to me to leave the ambiguous cases alone when we can easily make a huge difference with simple cases.

Posted by matthewvosper 8 months ago

I was thinking more of persuading them to split into two observations, one for each gender, with reference to new/current observations rather than legacy.

Posted by rkl 8 months ago

An alternative pathway would be to follow iRecord into having a multiple gender category for an observation.

Posted by rkl 8 months ago

It is possible to record multiple sexes in iNat just not with the sex annotation, but iRecord won't download it anyway. As far as improving iNat data is concerned the 'sex' observation field has 'mixed' as an option - it just won't help the recording scheme because they won't see it. Persuading people to split observations might work, if people were made to feel positively about getting two different things. I'd be a little wary in case people felt put out though. Roger and Stuart can always add 'multiple gender' in iRecord when it gets there if they want. Although we can improve things a lot, I don't imagine we can solve everything for them.

Posted by matthewvosper 8 months ago

Mmmm. That sounds a bit like being driven by pre-existing constraints. At least trying to get iNat to add 'multiple gender' to the sex annotation (which I assume passes to iRecord) looks to me like a lot of gain in terms of iNat/iRecord interoperability with very little pain, although there may be issues of which I'm unaware.

Posted by rkl 8 months ago

I agree, it would be good if the sex annotation could accommodate multiple sexes and mating pairs. Even if they did I presume it would still require BRC to update the code to download it. There is a (very long) conversation about annotation requests here. I think most resistance comes from the fact that iNat would need to change the definition of an observation (interaction with an individual organism) to implement it, but I for one would vote for such a feature request. I don't foresee it though.

Posted by matthewvosper 8 months ago

If accommodating the import of an existing concept into a (hopefully) controlled-vocabulary constrained metadata field were to require a code update then I would despair. Hopefully CEH took that much on board from our NERC data centre interoperability work 15 years ago! I can understand the iNat unease about observation entity definition creep, but if they are really worried they should be taking steps to make mating pair and multi-gender photo set observations invalid. If they don't want to do this - which to me is sensible - then their metadata model needs the necessary flexibility to cope. When I was working our data and metadata models had to accommodate many unforeseen use cases and frequently the only way was to broaden (but NEVER narrow or delete) the definitions of concepts in controlled vocabularies. Such changes were never taken lightly and were usually discussed by a group of experts before implementation.

Posted by rkl 8 months ago

Forgot to say. If this project takes off - as I believe it will - then it could well be a tool for getting iNat to change.

Posted by rkl 8 months ago

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