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What are scorpions?

Why study scorpions?
Life History
Dwindling Expertise

Determining the Age of Scorpions

 Telling the absolute age of a scorpion is not possible, but the relative age can be determined reasonably well.  The ability to distinguish juveniles from adults is particularly important because species identification is much easier with adult material.  The characters offered herein that purpose are, in part, subjective, and should be used in combination with each other.  With practice, few errors are likely to be made --  particularly once the observer becomes familiar with the species in question.           

Body size is an obvious criterion to use in determining the age of a scorpion, but should be utilized only with sufficient knowledge of adult size variation for a given species or population.  The reason for this is that individuals of many species mature at different instars (stages), and there may be significant interpopulation differences in adult body size.  Nevertheless, in many cases, body size is about the only way to distinguish adult females from juvenile females without dissection.           

Pectinal development can be important in age determination of males.  With the last molt, the pectines and pectinal teeth show distinct enlargement, and the pectines usually become splayed out.  In subadults, they are noticeably smaller and tucked in against the coxae of the 4th pair of legs.  In addition, the genital papillae of adult males are larger and tend to protrude more from underneath the genital opercula.  Simultaneous study of subadult and adult males aids in learning to distinguish adult males from subadult, and will train the eye to make the correct maturity determination when only a single specimen is in hand.           

The easiest way to determine if a female is an adult is to note whether she is gravid.  Gravid females can be quite plump; however, plumpness alone is no sure sign that the female is carrying embryos.  Well-fed specimens may look quite gravid when they are not.   The only way to be sure that a female is gravid is to cofirm that the embryos are visible as whitish, oval-shaped objects through the pleural membranes of her abdomen.           

Coloration also provides important clues to maturity in many species.  In general, very young juveniles of many species are pale yellow.  However, even mid- to late-instar juveniles of Diplocentrus spp., Uroctonus apacheanus , and Vaejovis intermedius are quite differently-colored than adults.  As these young scorpions age, their color gradually darkens, and it is not until the final molt that more drastic darkening occurs.  In U. apacheanus, subadults are generally yellow brown to light brown, sometimes with the carapace a little darker.  With attainment of adulthood, the carapace, pedipalps, and metasoma (and sometimes the entire dorsum) becomes dark reddish brown.  The same is true in members of Diplocentrus, except that the mid- and late-instar specimens begin to acquire varying degrees of the darker colors (which still appear somewhat faded).  In  Vaejovis intermedius, late-instar juveniles sport a light yellow-brown base color with a pair of dusky stripes down the middle of the back; the pedipalps and posterior metasomal segments are orange.  With the adult molt, the stripes are lost, the body becomes a grayish brown, and the orange areas turn dark reddish brown.  Color changes also occur in other species, but in most are more subtle.           

With preserved specimens, it is always possible study the reproductive anatomy directly through dissection.  Adult males will have well-formed hemispermatophores (the precursor to the spermatophore described earlier) and testicular tubules, and females a well-developed system of ovariuteral tubules, occasionally with developing embryos.  This dissection can be done by slitting the lateral membrane  on one side lengthwise from just behind the carapace to the vicinity of the seventh mesosomal segment.  Using a pair of forceps, the slit can be held open and one can peer inside to confirm the presence or absence of the reproductive organs.  In this way, the specimen can be partially dissected without completely destroying it.



The material included in this site is based upon work supported by the National Science Foundation under Grant No. 0413453.  Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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