Wittmer W. The genus Phengodes in the United States (Coleoptera: Phengodidae). Col. Bull. 29: 231-250, 1975. https://www.jstor.org/stable/3999661
I transcribed a bit of this, visible in the preview. If I read this correctly, the author is talking about identifying adult males only. (Anybody who has seen the original, please correct me if I'm wrong.)

Abstract
The species of Phengodes occurring in the United States of America are revised. Based on the structure of the maxillary palp, Phengodes is divided into Phengodes s. str. (type-species plumosa Ol.) and Phengodes (Phengodella) subg. nov. (type-species frontalis Lec.) The author examined 10 species and 4 subspecies. The following are described as new: arizonensis, fenestrata, fusciceps intermedia, inflata, laticollis meridiana, nigromaculata, and mexicana. P. sallei Lec. (1881) is synonymized under fusciceps Lec.
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The genus Phengodes Ill., wide ranging in the United States, has never been studied in detail. The few descriptions are, in most cases, based upon single specimens, some ow which have insufficient locality data.
I have examined the types of all species, even that of Phengodes plumosa Oliv. (the type-species), with the only exception of P. floridensis Blatchley. Even thus it has not in all cases been easy to determine the species. Generally, the species are rather uniform in size, coloration, and shape, especially when specimens from the same locality are compared. The interocular distance and the length of antennal segments 4 to 6 have been useful for distinguishing species...