Research Article |
Corresponding author: Justin O. Schmidt ( ponerine@dakotacom.net ) Academic editor: Maksim Proshchalykin
© 2021 Justin O. Schmidt, Li S. Schmidt, Deborah K. Schmidt.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Schmidt JO, Schmidt LS, Schmidt DK (2021) The paradox of the velvet-ant (Hymenoptera, Mutillidae). In: Proshchalykin MYu, Gokhman VE (Eds) Hymenoptera studies through space and time: A collection of papers dedicated to the 75th anniversary of Arkady S. Lelej. Journal of Hymenoptera Research 84: 327-337. https://doi.org/10.3897/jhr.84.68795
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Female diurnal velvet-ants are among the insects most strongly protected against predation. These protections include an exceedingly hard and tightly fitting integument, powerful legs and rapid running ability, aposematic color, aposematic warning stridulation, aposematic odor, and, most famously, legendarily powerful stings. Small to medium-sized velvet-ants were rarely injured or killed in tests against potential predators. Velvet-ant stings are painful, often exceedingly so, but have low toxicity or lethality and are of defensive value primarily by causing intense pain. Despite being well defended against predators they do not form night-time resting aggregations that might enhance mutual protection.
Dasymutilla vesta, Dasymutilla bioculata, predation, venom, aggregation
Velvet-ants are a successful family of more than 4300 species in a large number of subfamilies and genera (
Velvet-ants are also the poster children for uniqueness among the solitary aculeate wasps. Their most conspicuous difference from most other wasps is the total absence of wings in the females and presence of wings in most males (Fig.
The paradox of velvet-ants is why they are dramatically different from other wasps in the Vespoidea, despite being nestled deeply within the clade (
Velvet-ants were collected in southern Georgia and north and central Florida, USA during the summer of 1976. They were returned to the lab and housed in containers with a layer of sand on the bottom and provided honey and water ad lib. To determine if a velvet-ant were an acceptable prey to a variety of predators, a series of arenas with sandy floors was established. These ranged in size from 5×8 cm to 36×60 cm, depending upon the sizes of the predators. Tests were conducted by introducing the velvet-ant into an arena that already contained the potential predator. In most cases, the interactions were observed until the predator attacked or had displayed either avoidance or lack of interest. In some exceptional situations the animals were left together for longer periods of time. For example, tests involving velvet-ants and antlion larvae were conducted in the smallest 5×8-cm arenas in which the larva would make a conical pit-trap in the sand. The velvet-ant would often fall into the pit and escape from it repeatedly, thereby providing the antlion numerous opportunities to catch it. In situations where the predatory behavior was not evident, the predator was then provided an alternative, palatable prey and, if that prey item were attacked, the test was scored as no predation on the velvet-ant.
Because sting pain cannot readily be measured instrumentally or with great precision, a pain scale for the immediate, acute pain caused by a sting was developed (
To determine the lethality of the venom of velvet-ants, pure venom was obtained by the method of
An arena 60×60×4 cm LWH with washed grey sand to a depth of 7 mm and covered with a clear glass top was established in the laboratory in Athens, Georgia. The top edge faced the single window that provided natural sunlight. Refugia within the arena were 6×8-cm rectangles of 80-lb weight index-card material placed on top of the sand in the four corners, midway along the four sides, and with a final rectangle placed in the center of the arena. Thirty-six female Dasymutilla vesta (Cresson, 1865) collected in Union County, Florida and 12 female D. bioculata (Cresson, 1865) (
The outcome of challenges in which a very small, Pseudomethoca frigida (Smith, 1855) (4–6 mm long (
Trials of potential predators of small to medium-sized female velvet-ants.
Potential predator (Common name) | Velvet-ant species | n † | Result‡ | Comments |
---|---|---|---|---|
Vertebrates | ||||
Meriones unguiculatus (gerbil) | Dasymutilla vesta | 6 | 2+,4– | Grabbed, stung, released, repeat, eaten |
M. unguiculatus | D. vesta | 2 | 2– | Stinger experimentally removed; naïve gerbil grabbed and released; experienced gerbil avoided |
Mus musculus (mouse) | D. vesta | 4 | 4– | Minor interest, could not hold wasps |
Sceloporus woodi (Florida scrub lizard) | D. vesta | 2 | 2– | Grabbed, released, no further attacks |
S. woodi | D. vesta | 3 | 3– | Stinger removed; grabbed, released |
S. woodi | Pseudomethoca frigida | 2 | 1+,1– | Ate 3 P. frigida, avoided D vesta; the other lizard avoided P. frigida |
Anolis carolinensis (Carolina anole) | D. vesta | 1 | 1– | Stinger removed; eyed, but no attack |
Spiders | ||||
Aphonopelma hentzi (tarantula) | Pseudomethoca sanbornii | 11 | 1+,10– | 148 mg spider ate; 10 spiders 163–363 mg could not puncture |
A. hentzi | D. vesta | 11 | 9+,2– | Spiders 148–363 mg |
A. hentzi | D. vesta | 9 | 9– | Adult spiders; some grabbed, none injured |
Hogna carolinensis (wolf spider) | P. sanbornii | 5 | 5– | Adult spiders |
Insects | ||||
Myrmeleontidae sp. (antlion larvae) | D. vesta | 3 | 3– | Repeatedly grabbed but could not hold |
Myrmeleontidae sp. | P. frigida | 4 | 1+,3– | 3 Grabbed but could not puncture; 1 ate |
Most gerbils would release velvet-ants as soon as the latter stridulated. Some, however, persisted and two out of six individuals attacking D. vesta were stung and dropped their velvet-ant, only to grab her again, this time rapidly spinning her with the paws while biting. This action appeared to prevent further stings, allowing the velvet-ant to be consumed. Two of the gerbils that avoided velvet-ants after their initial encounters also the next day avoided velvet-ants whose stingers had been clipped to eliminate their ability to sting.
Florida scrub lizards avoided D. vesta irrespective of whether they had an intact stinger or not. In contrast, one lizard attacked and consumed three of the much smaller P. frigida. The one tested Carolina anole did not attack a D. vesta that had her stinger clipped.
Most small immature tarantulas (Areaneae:Theraphosidae) attacked, but did not succeed in injuring, the medium-sized P. sanbornii. Those same small tarantulas overwhelmed most of the smaller D. vesta. For unknown reasons, all nine adult tarantulas did not prey on their offered D. vesta.
Larval antlions readily grabbed and attempted to puncture offered velvet-ants. Only one individual succeeded in piercing and consuming a P. frigida, a much smaller species than D. vesta.
The sting pain level of most small velvet-ants such as Dasymutilla vesta, D. asteria Mickel, 1936, D. thetis (Blake, 1886), D. archboldi Schmidt & Mickel, 1979, and P. frigida was a 1 on the pain scale. Larger species including D. bioculata, D. gloriosa (Saussure, 1868), D. foxi (Cockerell, 1894) and D. sicheliana (Saussure, 1868) delivered a pain level of 2. Stings of the largest species such as D. klugii (Gray, 1932) were intensely painful and rated a level of 3. Sting pain levels of 3 are uncommon and is observed in only a few other species including harvester ants in the genus Pogonomyrmex and a few large paper wasps in the genus Polistes (
The published values for the venom lethality of D. klugii is 70 mg/kg (
The overnight resting locations of D. vesta and D. bioculata are shown in Table
The resting locations for D. bioculata were based on 10 replicates because on one day the survey was too late and many of the individuals were already active on the surface and that replicate was excluded. The results were similar to those of D. vesta in that significantly fewer individuals were found under the center rectangle than expected. Likewise, the combined four corners sheltered no different number of individuals than expected (χ2 = 2.85, P = ns). The combined four mid side rectangles sheltered marginally more individuals than expected (χ2 =6.05, P < 0.05).
To determine if the two species interacted with each other positively or negatively, the results of the 10 replicates in which both species were present at the same time were combined and analyzed (Table
Location | Light Lf corner | Light mid | Light Rt corner | Left mid | Dark Lf corner | Dark mid | Dark Rt corner | Right mid | Center | Sand |
---|---|---|---|---|---|---|---|---|---|---|
Dasymutilla vesta (replicated 11 times) | ||||||||||
Total | 40 | 40 | 37 | 37 | 32 | 47 | 39 | 46 | 23 | 50 |
Mean | 3.64 | 3.64 | 3.36 | 3.36 | 2.91 | 4.27 | 3.55 | 4.18 | 2.09 | 4.55 |
S Dev | 2.06 | 2.38 | 2.11 | 1.43 | 1.30 | 1.19 | 1.69 | 2.09 | 1.51 | 1.69 |
Chi sq | 0.14 | 0.14 | 0.01 | 0.01 | 1.53 | 4.94 | 0.02 | 3.06 | 10.93 | n/a |
Prob | ns | ns | ns | ns | ns | 0.05 | ns | ns | 0.005 | n/a |
Dasymutilla bioculata (replicated 10 times) | ||||||||||
Total | 9 | 11 | 10 | 15 | 7 | 13 | 10 | 12 | 2 | 30 |
Mean | 0.9 | 1.1 | 1.0 | 1.5 | 0.7 | 1.3 | 1.0 | 1.2 | 0.2 | 3 |
S Dev | 0.74 | 0.74 | 1.70 | 1.65 | 0.48 | 0.95 | 1.63 | 1.55 | 0.42 | 1.41 |
Chi sq | 0.03 | 0.08 | 0.00 | 4.30 | 1.15 | 1.38 | 0.00 | 0.52 | 11.04 | n/a |
Prob | ns | ns | ns | 0.05 | ns | ns | ns | ns | 0.001 | n/a |
Both species combined (replicated 10 times) | ||||||||||
Total | 44 | 45 | 45 | 49 | 36 | 55 | 45 | 54 | 25 | 76 |
Mean | 4.4 | 4.5 | 4.5 | 4.9 | 3.6 | 5.5 | 4.5 | 5.4 | 2.5 | 7.6 |
S Dev | 2.27 | 2.42 | 2.84 | 1.37 | 1.17 | 1.43 | 2.17 | 1.78 | 1.51 | 1.71 |
Chi sq | 0.00 | 0.00 | 0.00 | 0.83 | 2.70 | 4.78 | 0.00 | 3.89 | 15.85 | n/a |
Prob | ns | ns | ns | ns | ns | 0.05 | ns | 0.05 | 0.001 | n/a |
Female velvet-ants of all sizes appear to have few significant predators. The relatively small sized Dasymutilla vesta survived most attacks by gerbils, especially if they managed to sting the attacker. Gerbils are rodents that readily prey on insects and small invertebrates. Some gerbils learned from the encounter with D. vesta to avoid the species in a later encounter, even when the velvet-ant had been rendered harmless by modification of the stinger. Gerbils are intelligent predators and some learned to avoid stings by quickly rotating the insect with their paws while biting to inactivate her, a behavior that likely would also be useful in their native desert habitats of Asia where they are presumed to encounter stinging scorpions.
Florida scrub lizards grabbed D. vesta, then quickly released them unharmed. This apparently was not because of the sting as individuals incapable of stinging were also quickly released. Although Florida scrub lizards damaged no D. vesta, they did overwhelm three of the much smaller Pseudomethoca frigida. This might be because the integument of the smaller P. frigida was not as hard and could be crushed in the lizards’ jaws.
Spiders presented a mixed story. Some tarantulas could overcome and consume velvet-ants, whereas others would not attack them. Large wolf spiders (Lycosidae) would attack and then release velvet-ants. This appears to be the result of the vibration transmitted to the spider’s chelicerae by the stridulation of the velvet-ant (
Antlion larvae are omnipresent in the sandy areas frequented by velvet-ants in this study and could present serious risks. Despite falling into their conical pits numerous times in our tests, no D. vesta was ever successfully captured and eaten by a larva. One P. frigida that was grabbed was punctured and eaten. In the case of antlion larvae, the velvet-ants’ integumental hardness and the tight-fitting sclerites appear to be the key to their survival.
Despite their common name of “cow killers”, the stings of velvet-ants are not especially toxic to mammals. The most lethal venom of the five species evaluated was estimated 32 mg/kg for D. occidentalis. This lethality pales in comparison to those of the honey bee at 2.8 mg/kg, or 0.4 mg/kg for Pogonomyrmex harvester ants (
Velvet-ants are frequently encountered in good habitats in large numbers (
We thank Jillian Cowles for permission to use her photographs of Dasymutilla asteria and Denis Brothers and Chris Starr for helpful suggestions and manuscript review.