FNZ 9 - Protura (Insecta) - Morphology and Diagnostic Characters
Tuxen, SL 1986. Protura (Insecta). Fauna of New Zealand 9, 52 pages.
(
ISSN 0111-5383 (print),
;
no.
09.
ISBN 0-477-06765-4 (print),
).
Published 24 Feb 1986
ZooBank: http://zoobank.org/References/6B017956-58FF-4405-9795-98BA1D6FE682
Morphology and Diagnostic Characters
Figures 2-13 are scanning electron micrographs that show whole animals and important characters of Amphientulus zelandicus and abdominal tergite VIII of Gracilentulus gracilis. So as not to spoil the micrographs with labelling I have drawn a specimen in dorsal and ventral aspect (Figures 14 and 15) and marked the characters on them. Other characters are shown in more detail and at greater magnification in the taxonomic drawings (Figures 22 - 165).
The three tagmata - head, thorax, and abdomen - are easily distinguished. On the head the following characters are diagnostically important.
The pseudoculi (ps) are probably homologous with the postantennal organ in Collembola. They may be round or oval, with or without structures, or with a small or bigger "lever" posteriorly. The length of the head behind the rostral setae divided by the length of the pseudoculus without the lever is termed PR.
The mandibles (mdb) may be broad apically with from two to four minute teeth or taper into a pointed tip.
The maxillary palps (mxp) are three-segmented and end in a tuft of setae. The last segment but one carries two sensillae, the shape of which may be important.
The canal of the maxillary gland (mxgl) is a single tube in Eosentomoidea but is varied and systematically very important in Acerentomoidea. Originating from the gland as a variously shaped bulb, it is tubular proximally, and has a widening calyx before the final duct. The maxillary gland canal, originally called "filamento di sostegno", is of ectodermal origin and easily seen in cleared specimens. The length of its proximal part (i.e., proximal to the calyx) may be compared with the proximal branch of the fulcrum (fu), which is the tentorium-like internal skeleton of the entognathous head.
The labial palps (lbp) are small, usually one-segmented, ending in a tuft of setae like the maxillary palp or reduced, with three or four setae and an often sausage-like sensilla.
On the thorax the most important characters, besides the chaetotaxy, are associated with the foretarsus (see, e.g., Figures 22 and 23). This is generally long and slender, and terminates in a claw attached to a small pretarsus on which a ventral empodium (emp) and a dorsal sensilla (s) are attached. The length of the foretarsus proper (excluding pretarsus and claw) divided by the length of the claw plus pretarsus is called TR (tarsus ratio). The length of the empodium divided by the length of the claw proper is termed EU. The tarsus carries a number of sensillae usually with rounded tips, and a number of setae (with pointed tips) which are designated with letters and numbers and are of extreme systematic value.
Sensillae. There are three dorsally, tl, t2, and t3. On the exterior side there are at most seven, labelled a-g, in Acerentomoidea but an additional f2, x, y, and z in Eosentomoidea. On the interior side Acerentomoidea have at most three sensillae, a', b', and c', and Eosentomoidea have an additional b'2. Sensilla t1 may be club-shaped (claviform) or have a ball-shaped head (baculiform).
Setae. These are arranged in a certain pattern on the four "sides" of the foretarsus. Dorsally there is a zigzag line of seven setae in Acerentomoidea, alpha 1 - 7; and in Eosentomoidea an additional alpha 3' in line with alpha 3. Ventrally another zigzag line of seven setae, ß1-7, is found in Acerentomoidea; in Eosentomoidea the line continues up to ß9. Seta ßl is always very small. On the exterior side five setae, sigma 1-5, are found more or less in a zigzag line in both suborders. Finally, the four proximal setae on the interior side, sigma 1 - 4, are arranged in an oblique line beginning dorsally, while sigma 5 and sigma 6 are closer to the midline. In Eosentomoidea there are an additional sigma 3' and sigma 4' dorsal to sigma 3 and sigma 4. This sounds complicated, but in reality is usually easily recognised, and is important for the identification of the sensillae.
From the designation of the sensillae and setae it might seem as if the Eosentomoidea are more specialised than the Acerentomoidea, but I do not think they are. The reason is historical: the designations were at first applied to acerentomid species (by Conde and myself) and later transferred to eosentomids.
The position of tl is important, especially in Eosentomoidea; it is called BS, namely the distance to tarsal base divided by the distance to tarsal apex (without claw).
The middle and hind legs are of systematic interest only in connection with their claws: these are knife-shaped in Eosentomoidea, but in Acerentomoidea more or less boat-shaped. In Eosentomoidea the length of the empodium relative to the claw (EU) is of importance.
In Eosentomoidea the mesothorax and metathorax carry one spiracle each on either side, but the tracheae of each do not meet.
The abdomen consists of twelve segments, or eleven plus a telson. The anus opens on the telson, and the genital organs open between this and the eleventh segment. The following characters are of importance.
The first three abdominal segments each carry a pair of rudimentary legs (e.g., Figures 82-84). The first pair is always two-segmented, ending in a terminal vesicle; in Eosentomoidea it always has five setae, in Acerentomoidea never more than four. The second and third pair may be like the first (Eosentomoidea) or a rudimentary bud without a vesicle and with one, two, or three setae, one of which is always big, subapical, and lateral and the two others smaller, and lateral or medial.
The eighth abdominal segment carries posteriorly on either side the opening of the big abdominal glands. This opening is always covered by a "lid", which in Eosentomoidea is undifferentiated but in Acerentomidae is diverse, with or without teeth differing in number and length; it is often called comb VIII.
In Acerentomoidea a so-called "striate band" is present proximally on the eighth abdominal segment; it is most differentiated dorsally. In Protentomidae it is only a transverse line, but in Acerentomidae there are two transverse lines with or without structures in between. These structures may take the form of a cuticular grate of stakes (hence the name), but even where these are missing one may imagine their presence because corresponding subdermal structures are visible, probably glandular tubules. Even where the stakes are missing a cuticular structure may be seen (see Figures 10 and 11).
The external genitalia differ from those of all other insects. In both sexes they open ventrally between the eleventh segment and telson (the gonotreme). In both sexes there are a pair of basal arms and two styli. The male arms, or periphallus, carry setae but the female arms, or perigynium, never do. The gonopore is on the styli, and thus double, in the male but between the styli, and thus single, in the female. The male genitalia are generally alike in all species, though differing between Eosentomoidea and Acerentomoidea, whereas the genitalia show distinct specific differences in the female.
In Eosentomoidea the styli most often carry a curious structure, called the processus sternalis (plural: processus sternales), which is of characteristic shape between species and thus of great taxonomic importance. In Acerentomoidea there is no such structure; each of the styli ends in an acrostylus, the shape of which may also be of taxonomic value (see Figures 29 and 117; also Tuxen 1970, pp. 21-24).
Chaetotaxy. The position, size, and number of setae dorsally and ventrally on the thorax and abdomen are of the utmost taxonomic value. They were designated by me in 1949 and tabulated in 1964. It is true that some variability in number of setae may be found within the species (in 1961 I examined the intraspecific variability in 476 specimens of one species, Acerentomon gallicum Ionescu), but used with care, chaetotaxy is an important specifically distinctive character. In principle the chaetotaxy is as follows.
The pronotum almost always has four setae in a single row.
The mesonotum and metanotum each have an anterior row of equally long setae, a median pair of setae, and a posterior row with principal and accessory setae as on the abdominal segments.
Abdominal terga (Figures 16 and 17): the setae are arranged in two transverse rows on each side. The anterior row is designated a1-5, since the setae are generally of equal length. In the posterior row the setae are arranged into long, strong principal setae (p1 - 5) and much thinner accessory setae (p1' - 4'). The accessory setae may be longer or shorter than the principal ones, but are always more gracile, and may be placed just anterior to the hind border or in an excavation of the hind border. (They are designated as 1a, etc. instead of 1' by Imadate and others.) On tergites IX-XI only one row is present on each side.
Thoracic sterna: their chaetotaxy is not used in taxonomy, being uniform throughout the Protura.
Abdominal sterna (Figures 16 and 17): the setae on sternites I - VII are arranged into two rows, the posterior row with principal and accessory setae; but on sternites IX - XI there is only one row. The eleventh sternite always carries four fewer setae in the maturus junior than in the adult.
For descriptive purposes chaetotaxy can be tabulated into big schemata with every seta in every stage stated, or expressed according to "Tuxen's scheme", with the numbers of setae in the anterior and posterior rows given respectively above and below a line, and then notes about important occurrences or absences of single setae. The latter scheme is used in this contribution.