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: https://www.inaturalist.org/observations/identify?quality_grade=needs_id%2Cresearch&taxon_id=49995&place_id=6857&without_term_id=9
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 June 28, 2023 01:53 PM by matthewvosper