Phloeodes diabolicus vs. Phloeodes plicatus

The ranges of P. diabolicus and P. plicatus overlap so they are often confused with eachother. The first thing to look for is the overall appearance at first glance. The structure of P. plicatus is very blocky and angular compared to P. diabolicus' structure which is more rounded and smooth. The blocky appearance of P. plicatus is a resultant of a group of tubercles located near the elytral tips (seen below). P. diabolicus lacks these tubercles and instead has a single raised portion on its elytra. 

P. plicatus is often smaller than P. diabolicus being around 18 mm compared to around 25 mm. However, P. diabolicus has been recorded as small as 15 mm and P. plicatus has been recorded as large as 22 mm.

The coloration of P. plicatus is usually a tan to brown color whereas P. diabolicus is usually black-blue with patches of black setae and occasionally patches of yellow setae. P. plicatus lacks these patches.

Another major difference is the antenal cavities. P. plicatus has an under-developed, incomplete antenal cavity while P. diabolicus has a full, complete cavity. This can be seen below.