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Basic Anatomy of a Scorpion

Heterometrus laoticus

The basic external morpholgy of most scorpions includes:
1) 4 pairs of legs
2) 2 pincers known more correctly as pedipalps
3) Chelicera which are the appendages which brings food to the 'mouth'
4) Prosoma which is the 'head' containing the eyes if present. More commonly referred to as cephalothorax
5) Opisthosoma which is the abdomen. Contains 7 mesosomal segments, 5 metasomal segments (tail) and the telson (sting)
6) Pair of pectines .
7) Genital operculum
8) Sternum
9) Metasoma

Heterometrus laoticus (ventral) Closeup ventral view of a dead
Heterometrus laoticus with labels
of the important parts

For those who wants to know more

Legs
Till now terminology of the various sections of the appendages is still not standardised. Generally, the leg is divided into:
coxa (closest to sternum), trochanter, femur, patella, tibia (with tibial spur for some family), basitarsus (with pedal spur distally for some) and tarsus. It ends with the pair of ungues (lateral claws) which has apotelle and dactyl (median claws) ventral to it.
Used mainly for locomotry function and may be used to dig substrate and move baby scorpions (for females)

Pedipalps
The pedipalp is divided into:
coxa, trachanter, femur, patella, tibia and tarsus. The tibia and tarsus forms the chelae of the pedipalp. The pedipalp contains many hairs (setae). It is divided into 3 types of which; one type is sensory and named as trichobothria. The arrangement and number of trichobothria is unique to each genus and is used extensively to identify scorpions. The trichobothria are erectile and unidirectional in mobilty so that combined information of differently aligned trichobothria gives a spatial perception of disturbance in air current.
Used to grasp prey and for defense against predators. Also employed to grasp female chelae while mating.
Peculiar fact: No abduction muscle for tarsus. The abduction (for opening) of tarsus is thought to be partially due to increased hydrostatic pressure from dorsoventral contraction of opisthosoma. It's abduction is now thought to be mainly from the elastic nature of the fibres in the infolded corium joining the tibia and the tarsus.

Pectens
Also known as the pectines. It is a peculiar gill like structure which some postulated to be remnants of the ancestral gills of the Silurian water scorpions.. This however has not been substantiated and pectens apparently is also present in euryterids (ancestral scorpions). It is found to be of sensory function. In most scorpions, it has mechano-receptors which is hypothesized to help the male choose suitable substrate for depositing spermatophore and sense surface vibrations. In some species, it is found to have some contact chemoreceptors. It is one of the most convenient means of determining sex. of some species.

Prosoma
Some wonder why the eyes are not included as part of the external morphology. Reason is simple. In cave dwelling (triglobitic) species like Sotanochactas elliotti , eyes are absent. In some other species, there can be up to 6 pairs of eyes! Besides the central eye group in the picture above, there may be a lateral eye group in the anterior lateral part of the prosoma. Also note that the chelicera is not its mouth but an appendages to aid feeding and grasping of food.

Chelicera
Divided into 3 segments: coxa, tibia (fixed finger), tarsus(movable finger). Used to grasp and crush prey before sucking it. Curiously, it does have an abduction muscle which is absent in the pedipalp. Pattern and shape is used extensively in differentiating the families.

Opisthosoma
The ventral carapace(sternite)and the dorsal carapace (tergite)of the mesosoma is non-continuous while that in the metasoma (tail) is. . They are joined by a whitish membrane called pleural membrane which would be stretched when the scorpion is very full or pregnant. The mesosoma is divided into 7 parts while the metasoma which becomes longer distally has 5 parts.

Sternum
Sternum is the junction where the coxa of most legs meet. The relative length and shape of the sternum is used extensively to differentiate the families. Pentagonal sternum is thought by some to be of a more primitive origin.  Triangular sternum is characteristic of buthids which includes some of the more dangerous species.

Genital Operculum
Genital operculum covers the reproductive organs of the scorpions (genital orifice of females). In the male, the genital operculum is completely separate. A pair of genital papillae may protrude from the posterior part of the operculum for males of some species. This may be sclerotised or otherwise but is less evident in some buthids. This is another key sex dimorphism region.

Metasoma
The metasoma or the so-called 'tail' is divided into 5 true segments. Distal to the metasoma V is where the telson(sting) is. There are generally 2 venom glands under voluntary control within the vesicle of the telson. The telson ends with a hypothermic needle like sting known as aculeus. I re-emphasise that telson is not a true segment and does not belong to the segments of the opisthosoma. The telson itself is divided into two parts; the vesicle and the aculeus. In some species they are 'separated' by the subaculeus tubercle. This, however, is not present in genus Scorpionidae to which Heterometrus sp. belongs to.