Authors should consider illustrating two or more specimens of a species when trying to communicate the nature of some morphological trait in that species. Here’s why.
In illustrating morphological features in a publication, authors typically choose one specimen to image from each species. For example, with male genitalia of Bembidion, one typically shows a plate containing a single genitalic image of each species. Consider two species of Bembidion subgenus Semicampa from North America, B. roosevelti and B. rubiginosum. These are small brown beetles, extremely similar to one another, which have been confused in the past. Lindroth’s (1963) concept of B. roosevelti is a mixture of the two species; Bousquet and Webster (2006) speculated that they may be synonyms. However, if you dissect genitalia, you will see subtle differences between the two species (see below).
But of the many differences that can be seen in the two pictures above, which ones represent differences between species, and which are nothing but vagaries of the preparation, or lighting, or individual variation? That is, what is signal and what is noise?
If one had the time to do a pen and ink illustration, then one could examine multiple specimens per species, harmonize the variation, and illustrate an average specimen for each species, in the process stripping away some of the noise. That would allow one to better show the signal, but it wouldn’t be easy to convey the noise. With photography, it isn’t as simple to portray an average specimen. One approach I have used to conveying the signal and noise in photographs is to show multiple images per species, and to allow the reader’s mind to process and discover the relevant differences.
This is costly in terms of space on a printed page, but it can be worth it if it conveys a message that a single illustration doesn’t. Adding some arrows or other labels can help, too. Arrows in the following version show some features to compare between species:
The ability to show many illustrations is one of the advantages of publishing taxonomic data on the web, where space is cheap.
This reminds me of the saying “a person with one watch knows what time it is; a person with two watches is never sure”. In the case of illustrations, imaging more than one specimen of a species allows one to see which aspects vary, and that is invaluable.