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Species Castolus ferox

Assassin Bug - Castolus ferox - male Assassin Bug - Castolus ferox - female Rocconota? - Castolus ferox - female Assassin Bug - Castolus ferox - female Castolus ferox - male A True Bug - Castolus ferox A True Bug - Castolus ferox Castolus ferox? - Castolus ferox
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Hemiptera (True Bugs, Cicadas, Hoppers, Aphids and Allies)
Suborder Heteroptera (True Bugs)
Infraorder Cimicomorpha
Family Reduviidae (Assassin Bugs)
Subfamily Harpactorinae
Tribe Harpactorini
Genus Castolus
Species ferox (Castolus ferox)
Explanation of Names
Castolus ferox (Banks 1910)
Remarks

Yes, there is sexual dimorphism in C. ferox. The two (most obvious) dimorphic structures are the antennae and the connexiva. Regarding the antennae, there are some reduviid genera (harpactorine only?) in which the males have the third segment gradually thickened to the apex. Thus, the first thumbnail is a female (of course, I've examined this specimen) and the second is a male. Off the top of my head, I know that this morphology also is found in Atrachelus (although we only have the one image in the guide, which I think is a female). For the second character, female reduviids are larger than the males (like in many other groups). I think this may be evidenced in the robustness of the abdomen here, but I'm not 100% this will always be the case. Plus, you have other complicating factors (ie gravid females, malnourished individuals, etc.). I will keep an eye on this, using the antennae for a guide, to see if it really holds.

In the United States, C. ferox will really be the only species to consider. Froeschner (1988) reported the Mexican C. subinermis from Arizona, but I have yet to find the source for this record. I also have never seen specimens of C. subinermis, although the two species are easy to separate (long humeral spines in C. subinermis) so there is no question of identity in most cases.

Disclaimer: This next bit is just to cover my bases; I understand you may not actually be referring to these details. There also is a photographical element at work in these images. In the first thumbnail, the antennae are directed up toward the photographer, making the scape, in particular, look shorter than it really is. The second photograph was taken from an dorso-caudal view rather than a straight dorsal view. This distorts the proportions of the insect and as a result, it looks shorter, particularly in the area farthest from the photographer (ie head, pronotum).
… drswanny, 3 August, 2012