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Meloe (subgenus Treiodous) sp.
A large, 3 cm long, Oil Beetle of the Family Meloidae. Both males and females have large unwieldy abdomens, although females are larger than males. This one is probably a female and she unfortunately has suffered the loss of most of both antennae. They are completely flightless, lack underwings and have the elytra reduced to short little decorations. This one has very small elytra. They are called Oil Beetles because like most Meloids, they can excrete toxic cantharidin (found in their hemolymph) through their body joints which can blister human skin. When I put her in a jar, she excreted copious amounts of oily orange hemolymph. This one is all dull black with very little sculpturing on the body. The underside of the prothorax is bright red (see third picture). See spotting by Billo from Xalapa, Veracruz, Mexico which seems to be the same species (https://www.projectnoah.org/spottings/20...) and has both it's antennae. I think both of these might be either Meloe laevis or Meloe gracilicornis (https://bugguide.net/node/view/680241).
Found walking in the street at night, semi-rural residential area, San Cristobal de Las Casas, Chiapas, Mexico 2,200 meters.
These beetles have a fascinating and complicated life cycle: after mating, the female will dig a tunnel in the ground and lay up to 1000 eggs. The larvae which hatch are long-legged and very active. They are called Triungulins and are actually an extra larval stage not found in other beetle's development. These tiny triungulins climb up on flowers and wait for a solitary bee. They climb on and are carried back to the bee's nest where they molt to a normal fat immobile larva and proceed to eat the eggs and food sources which the bee left in her nest. They feed and molt through the normal larval stages, eventually pupate and new adults emerge. These females are not like the expanded gravid females often seen in Leaf Beetles. They are not "expanded" because they are gravid, so the exoskeleton is not stretched or distorted. There are at least 32 species of Oil Beetle. https://www.insectidentification.org/ins... https://www.insectidentification.org/ins... https://www.buglife.org.uk/sites/default... http://www.bbc.com/earth/story/20150506-... http://www.realmonstrosities.com/2014/06... http://www.wildlifeinsight.com/3771/big-... https://en.wikipedia.org/wiki/Meloe https://www.insectidentification.org/ins... https://thesmallermajority.com/2012/10/0... https://www.naturepl.com/stock-photo-oil... http://mobugs.blogspot.com/2010/04/ameri... https://naturehasnoboss.com/2018/05/14/b...