Identifying insects

This article is “work in progress” so is subject to change/improvement at any time …

One of the most fundamental things you will do with a specimen is to try to identify it – without a name the specimen has no point of reference. This article is a few words of advice based on my own experiences. Obviously, it is biased heavily towards calyptrate Diptera and parasitic Hymenoptera! ๐Ÿ˜‰

What is the best guide book?

Short answer

Probably “A Comprehensive Guide to Insects of Britain and Ireland” by Paul Brock.

Long answer

Probably “A Comprehensive Guide to Insects of Britain and Ireland” by Paul Brock, but with a very big caveat because it isn’t in any way comprehensive in terms that most entomologist would understand because it is missing thousands of UK species. This is no reflection on the author who is an excellent entomologist but I guess the publishers thought it needed a punchy title. That said, the book itself has a lot of excellent photos and covers more species than any other field guide and it would literally be impossible to have photos of all UK insect species because there are too many and most of them need microscopes, specimens and very specialist literature to identify them.

The Collins field guides are also tried and tested alternatives but, as with all field guides, they are missing huge numbers of species and they don’t always make it clear enough that, even if your insect looks exactly like the one in the picture, there are probably dozens of others that would also look identical to those pictures.

The truth is that insect identification is complicated because there are a lot of UK species and they are usually quite small. It’s also not always obvious how to identify something because some species are very variable in size and colour and the features used to prove the ID are very small and insignificant. The following rough groupings are just my own interpretation and the headings etc are off the top of my head:

  • Beginner: Butterflies, dragonflies, grasshopper & crickets. Novices can try these groups with a decent, picture-based identification guide that lists all the UK species. You shouldn’t need a hand lens and usually you can identify living insects from photos or in a specimen pot.
  • Intermediate: large moths, social bees & wasps, most hoverflies & soldierflies, insect galls. If you join some specialist Facebook groups, network with experts and go on a few free courses / events then anyone can start to work on these groups. In a year or two you can become fairly confident but it will still take lots of study to become an expert. In most cases you’ll need specialist books and a good hand lens to do the more difficult species.
  • Expert: micro-moths, mayflies, caddisflies, lacewings, solitary bees & wasps, sawflies, beetles, true-bugs, most other flies. To work on these groups you really need a microscope, to take specimens and to use more specialist but fairly accessible literature. You’ll need to network with many national experts but you’ll get plenty of help along the way. These are very rewarding groups to get into. The UK species are all fairly well known but you can make some very useful contributions to our knowledge of their distributions and ecology. You’ll possibly find new species to the UK and definitely extend their known ranges if you are recording outside the south of England.
  • Specialist: parasitic wasps, microscopic insects like thrips and the smaller insects of the intermediate & expert groups above. These groups take decades of dedicated study and usually require regular access to only a few national / world experts and reference collections. The literature is often very hard to locate, very old or missing completely. Many groups have no published keys or the groups are identified from “test keys” and notes distributed by experts so a lot of detective work will be needed to amass useful libraries. In these groups you’ll almost certainly encounter new species to the UK and potentially new species to science.

Of course, there are easily identifiable species in nearly all of the “difficult” groups and if you post good, close-up photos taken from lots of angles, you should be able to get help from experts on Twitter or specialist Facebook groups.

What is taxonomy?

Taxonomy is organising, categorising and naming the natural world. Just as we store our documents in folders, we also group types of wildlife together according to how related they are to each other and these groupings or ranks are hierarchical. We are all familiar with a species but this is just one level of categorisation:

Species sapiens our species – human beings
Genus Homo there are no other living species in this genus but there are many fossils of our close ancestors
Family Hominidae human-like apes
Order Primates apes and monkeys
Class Mammalia all mammals
Phylum Chordata animals with backbones
Kingdom Animalia all animals


Identifications are more correctly called determinations because we determine what something is. It is worth first discussing what we mean by a determination because many novices start with a few misconceptions about determinations and even about what we actually mean by a species.

A determination is just the opinion of one person at one time – which is why we put the name of the determiner and the year on the det-label. As we gain experience we often redetermine specimens and might come to different conclusions … we are only human. So, if you are sure that you have done your best and you have arrived at a determination then put your name on it and move on – don’t be afraid to be wrong – because we all make mistakes.

That said, try to arrive at a balance and do not be tempted to push a determination further than you are confident to go. It’s perfectly acceptible to say “I don’t know” or to just give the specimen a family / tribe or generic determination. As you gain experience you can always go back over those specimens and determine them further at a later date.

Next, when you write your determination make sure that you understand what you are saying.

  • Tachina fera (female)” then you are saying that you have examined it and you are confident about the identification down to species & sex level.
  • Tachina sp.” says that you are confident of genus but not about species.
  • Tachina cf. fera” means that you are sure that it is Tachina but you just think that it might be fera and you need to check it later … c.f. is short for the latin word confer, which means to compare. So it is like writing “Tachina (compare against fera)”.
  • Exorista rustica (group)” some times also “agg.” for aggregate. This means that you think it is part of a very closely related group of species, which usually require very detailed examination – usually of genitalia – which you haven’t had time to do or you might have a sex that is unidentifiable beyond this point.
  • Sarcophaga s.l.” means that it is Sarcophaga “in the broad sense” (sensu latu). Sarcophaga is a very large genus made up from many sub-genera and sometimes it is not possible to take an determination much beyond the fact that it is the main genus. The opposite of this is sensu strictu (which means in the strict sense of the word) so here you are saying that it isn’t just Sarcophaga but you are confident that it is “Sarcophaga (Sarcophaga)”.

So, in summary … do your best work … be confident … but do not push it too far and be very specific about what you are saying in your determinations ๐Ÿ˜‰

Using keys

Keys are the commonest and most reliable tool that you will use to achieve determinations. They are a logical and progressive way to arrive at a determination but you do have to be aware of a few pitfalls that you should avoid on the way.

First make sure that you are really familiar with the terminology that the key uses. There should be diagrams that illustrate the various body parts and it helps if you can quickly run through them and check that you can find the different regions of the body on your specimen. Also, there will be comparative words such as: anterior (the front); posterior (the rear); dorsal (the upper surface) ventral (the lower surface); proximal (the end closest to the body); distal (the end furthest from the body) etc.

Each numbered section in a key usually comes as a pair of groups of questions – each being mutually exclusive (you hope!). This sounds pretty fool proof because you are shown 2 choices and you just have to choose which one most accurately describes your specimen and then jump to the couplet number it tells you to go to. Be very careful though because it is very easy (in your haste) to read both questions, understand them, decide which is best … and then choose the wrong option … believe me, this is a very common mistake.

To help you make the correct choice I usually underline the operative words of phrases in each couplet-pair … “does” or “does not” … “hairy” or “bare” and this can help to focus the mind at a critical point.

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