Seira dowlingi

Soto-Adames, Felipe N., 2008, Postembryonic development of the dorsal chaetotaxy in Seira dowlingi (Collembola, Entomobryidae); with an analysis of the diagnostic and phylogenetic significance of primary chaetotaxy in Seira, Zootaxa 1683, pp. 1-31 : 4-19

Seira dowlingi

No scales are present on first instar larvae.

Scale distribution on antennae. Two scales appear on the dorsal base of Ant. I–II during 2 nd Inst.; there are no scales on Ant. III–IV or on the ventral side of any segment. The distribution of scales remains unchanged during 3 rd Inst. and expands minimally at 4 th Inst. By 5 th Inst. scales cover all the dorsal surface of Ant. I, the basal half of Ant. II, and the basal fifth of Ant. III. In adults, two thirds of the dorsal surface and somewhat less than half of the ventral surface of Ant. II are covered with scales; Ant. III has scales dorsally on the basal fourth and ventrally on the basal fifth; scales are absent from Ant. IV and the ventral surface of Ant. I.

Ant. I botriothrica and sensilla. The first instar has three ciliate setae and is devoid of botriothrica (Fig. 13). The number of setae and sensilla increases during 2 nd - 3 rd Inst. (Figs 14, 15), but no botriothrica appear until 4 th Inst. (Fig. 16). A second botriothricum is added during 5 th Inst. (Fig. 18). Adults appear to be dimorphic in the number of botriothrica: males have four, females three (Figs 19, 20).

Ant. II botriothrica and sensilla. The first instar (Fig. 9) has one botriothricum and some ciliate setae, but no other sensilla. At 2 nd Inst. (Fig. 10) the number of ciliate setae increases, and a thin-walled, tapered sensillum is added. A second botriothricum appears at 3 rd Inst. (Fig. 11). The third botriothricum is added at 4 th Inst. (Fig. 12). In subsequent instars the relative distance between the three botriothrica changes, but only ciliate setae and sensilla are added. Short, conic setae associated with the botriothrica appear by 5 th Inst. (arrows in Fig. 17), increasing numerically during subsequent instars (Fig. 21).

Ocular setae. Most adults have five setae in the eyepatch, probably homologous to setae p, q, r, v, and t in Lepidocyrtus ( Mari Mutt 1986 a). Seta s is asymmetrically present in one specimen ( Fig. 27 ). Setae p, r, and t are primary (Fig. 24). Additional setae first appear during 4 th Inst., but are unstable: of seven specimens examined four have only the primary setae, one specimen adds v, but two others add q instead. All setae are present in all specimens beginning at 5 th Inst.

Labium. In the labial triangle of 1 st and 2 nd Inst. (Fig. 23) the anterior setae are smooth, the posterior setae are ciliate, and seta r is absent (M 1 oEA 1–2). Only the presence or absence of setae M 2 and r varies during postembryonic development. In 3 rd Inst., one specimen has both setae; five specimens in 4 th Inst. have M 2 r, one has only r (short, smooth and conic) and one lacks both. The final chaetotaxy is reached at 5 th Inst., all anterior setae are smooth macrochaetae and the posterior setae, except r, are ciliate macrochaetae (M 1 M 2 r E L 1–2 A 1–2). All adults have the complete chaetotaxy.

Scale distribution on legs. The pro- and mesothoracic legs lack scales during 2 nd Inst., but there is a single scale, distally on the external side of the metafemur and metatibiotarsus. Scales appear on the meso- and metacoxae in 3 rd Inst. By 4 th Inst. there are scales on all segments of all legs, but on the first two pairs the scales do not cover all surface. During 5 th Inst. all legs have a similar density of scales.

Scale distribution on collophore. Scales (2–3) first appear laterally on the anterior face during 4 th Inst. By 5 th Inst. both faces of the collophore have scales, concentrated on the central region of the organ, but they are more dense on the anterior face. The density of scales increases on both faces in adults (with respect to 5 th Inst.), but the density remains higher on the anterior face.

Scale distribution on manubrium. The dorsal manubrial face lacks scales in 2 nd Inst., but on the ventral face there is one line of scales along the lateral margins. The density of ventral scales increases with every instar, creating a uniform cover, but dorsal scales do not appear until 5 th Inst. Adults have one line of scales along the margins of the dorsal face.

Scale distribution on dens. During 2 nd Inst. scales cover two thirds of the ventral dental face, and by 3 rd Inst. they reach the base of the uncrenulate area anterior to the mucro. No scales are present on the dorsal dental face of adults.

Dorsal head chaetotaxy. The five rows of anterior setae (An, A, M, S, and Ps, Fig. 1 ) are already present in 1 st Inst. (Fig. 24). Rows S and Ps have the full complement of setae already during 1 st Inst. Post-embryonic changes on these rows are limited to the displacement of setae with respect to each other or their transformation into macrochaetae. The macrochaetotaxy of the posterior region is weakly developed in S. dowlingi ; adults have four rows of setae and all are present in 1 st Inst.

Row An. Adults have six to nine setae on this row ( Fig. 27 ) but only An 1, 2, 3 are primary (Fig. 24). An 1–3 are microchaetae during 1 st Inst., but are transformed into acuminate macrochaetae by 2 nd Inst.. A fourth macrochaeta (An 3 a) appears between An 2-3 at 2 nd Inst. (Fig. 25). Microchaeta An 2 a and a macrochaeta anterior to An 3 a appear by 3 rd Inst. (Fig. 26), but they are not stable; the seta anterior to An 3 a is missing in four specimens of seven studied, whereas An 2 a appears as a microchaeta or a scale. Both setae are fixed at 4 th Inst. A smooth microchaeta posterior to An 3 appears at 4 th Inst., but it is not fixed, remaining variable in the adults ( Fig. 27 ). All additions of macrochaetae to this row subsequent to 5 th Inst. occur between An 2 and An 3 a and seem to be added from posterior to anterior. The number of macrochaetae in this region is variable in the material studied, varying from one to three at 5 th Inst. and three to four in adults. The microchaeta antero-internal to An 3 appears after 5 th Inst.

Row A. Adults have four paired (A 2–5) and two unpaired (A 0–1) setae in this row. Setae A 0–3 and A 5 are primary. Secondary seta A 4 (Fig. 25) appears at 2 nd Inst.

Row M. Adults of S. dowlingi have setae M 2, M 3, M 4 i, and M 4 ( Fig. 27 ). During 1 st Inst., M0, M 2, M 3, and M 4 are microchaetae (Fig. 24), but M 1 is absent. At 2 nd Inst. seta M0 disappears (is not present even as a scale) and M 4 i appears (Fig. 25). M 4 becomes an acuminate macrochaeta at 3 rd Inst. (Fig. 26). There are no changes in this row after 3 rd Inst.

Row S. In adults of S. dowlingi this row includes nine macro- and two microchaetae arranged as 5 + 5 setae plus the unpaired S0 ( Fig. 27 ). At 1 st Inst. macrochaetae S 0–3 are arranged in two straight rows, with S 3 horizontally aligned to S 1. By 2 nd Inst. (Fig. 25), S 3 has moved posteriorly, closer to S 2 than S 1. S 5 remains a microchaetae in the adult. S 6 first appears as a macrochaeta in some specimens during 3 rd Inst. (Fig. 26) and is fixed by 4 th Inst. S 4 is absent in all instars.

FIGURES 9–20. Seira dowlingi ; 9–12, Antennomere II; 9, 1 st instar; 10, 2 nd instar; 11, 3 rd instar; 12, 4 th instar; 13–16, Antennomere I; 13, 1 st instar; 14, 2 nd instar; 15, 3 rd instar; 16, 4 th instar; 17, 5 th instar antennomere 2, arrows identify conic setae; 18–20, Antennomere I; 18, 5 th instar; 19. Adult male; 20, Adult female.

FIGURES 21 –26. 21, Seira dowlingi , adult antennomere 2; 22, S. blanca , adult antennomere 2; 23–26, S. dowlingi ; 23, Labial triangle, 1 st instar; 24–26. Dorsal head chaetotaxy; 24, 1 st instar; 25, 2 nd instar; 26, 3 rd instar.

Row Ps. Adults have 3 + 3 microchaetae, all primary. The only change during postembryonic development is a widening of the gap between Ps 3 and Ps 5 (cf. Figs. 24, 27).

Row Pa. Adults of S. dowlingi have one botriothricum, four macro- and one microchaetae; the botriothricum and four macrochaetae are primary (Fig. 24). The final chaetotaxy is completed at 2 nd Inst. (Fig. 25) with the appearance of microchaeta Pa 4. Macrochaeta P0 of Mari Mutt (1986 b) corresponds to Pa 5 in the present nomenclature system.

Row Pm. Adults ( Fig. 27 ) retain the number of setae found in 1 st Inst.

Row Pp. The organization found in adults is reached at 2 nd Inst. with the addition of microchaeta Pp 6 and macrochaeta Pp 7 (Fig. 26). One specimen at 2 nd Inst. has two (transient?) macrochaetae of unclear homology between Pp 4–5 (Fig. 25). Macrochaeta P 2 of Mari Mutt (1986 b) corresponds to Pp 5 in the present nomenclature.

Mesothorax. 1 st Inst. lacks setae m 3 and p 7 (Fig. 31); a 7, m 5, m 7, p 4, and p 6 are ciliate micro- or mesochaetae, all others are ciliate macrochaeta. Nineteen secondary setae are added by 2 nd Inst. (Fig. 32), most of them as ciliate macrochaeta, none of the seven primary microchaetae changes into a macrochaeta. There is much variation in the shape of setae in the posterior row, in area Pm. The most common arrangement in this area includes three groups (see Fig. 36 for group composition): group A has two macrochaetae (2 M), group B has two macro- and one microchaetae (2 M, 1 µ), and group C has three macrochaetae (3 M). Other arrangements observed among the ten individuals studied are (3 M) (1 µ) (3 M); (2 M) (1 M, 1 µ) (3 M); (2 M) (2 M, 1 µ) (4 M).

Instar 3 (Fig. 33) shows a general increase in the number of macrochaeta in the collar but only seven new setae appear posterior to the collar: macrochaetae a 5 p, a 5 i and p 1 i; and microchaetae a 2 p, m 4 i, m 4 ip and p 1 ip 2. The pattern in area Pm at 3 rd Inst. is (1 µ, 3 M) (3 M) (3 M).

During 4 th Inst. the collar retains the multiplet structure, but the homology of setae is difficult to discern, especially in the lateral area. Most specimens do not show the clear organization shown in Fig. 34. Five new setae are added, or change from µ to M, posterior to the collar (Fig. 34): macrochaetae a 5 i 2, p 1 ip, p 2 ea, p 3 p, and microchaeta m 1 i 2. The formula of area Pm is (1 µ, 4 M) (3 M) (5 M); variation was limited to PmC (4 M, 1 µ). The group of m 1–2 is unstable at this instar, m 1a2 is missing in three specimens, and m 1 a is a macrochaeta in two specimens. Other changes at this instar are the anterior displacement of m 4 p (cf. Figs 33–34) and the transformation of m 4 i into a macrochaeta. In 5 th Inst. the chaetotaxy of the area posterior to the collar is identical to that in adults, but the collar is only moderately more densely packed with macrochaetae than during 4 th Inst. (Fig. 35). The number of collar setae continues to increase in successive molts and the largest adults have considerably more setae than specimens in 5 th Inst.; the multiplet system is obliterated by the large density of macrochaetae in this and subsequent instars. Adults have (1 µ, 5 M) (3 M) (5 M) setae in region Pm (Fig. 36): four macrochaetae in groups m 1–2 and a 5, and three in group m 4. The base of m 1 is posterior to m 2; m 4 i is often posterior to m 4 p; and p 3, p 3 p and p 2 e form a densely packed diagonal.

Setae identified here as a 5 i and a 5 i 2 may be homologous to those included in generatrix a 4 and a 4 i in other entomobryids ( Szeptycki 1979), but their homology is unclear. Seta a 5 i may correspond to a 4 p 2, and a 5 i 2 to a 4 ip (cf. Fig. 34 and Figs 80–81 in: Szeptycki 1979), but by 5 th Inst. both setae are displaced towards the segmental midline and a 5 i is aligned with the multiplet a 4 i. In adults these setae are very distant from the collar. Since additional setae appear between a 5 i and a 5 i 2 and their presumptive generatix, it is impossible to establish a reasonable homology with apparently corresponding setae in adults of other genera.

Metathorax. The only primary setae absent at 1 st Inst. are m 2, m 3 and p 7 (Fig. 41). Seta m 1 and the supplementary seta external to a 7 (asterisk in Fig. 41) are smooth, other setae are ciliate. One of the seven individuals examined carried a supplementary seta of uncertain homology external to a 3 (arrow in Fig. 41). Seta m 6 p and macrochaeta p 2 a appear at 2 nd Inst. (Fig. 42). At this instar, m 1 moves to the inside of a line drawn between a 1 and p 1; a 4 and a 5 move posteriorly, just above the pseudopore and almost in line with p 3; a 6 also FIGURES 31–36. Seira dowlingi , mesotergal chaetotaxy; 31, 1 st instar; 32, 2 nd instar; 33, 3 rd instar; 34, 4 th instar; 35, Collar of 5 th instar; 36, Adult, excluding collar.

migrates posteriorly, aligning itself with p 5 and p 6. The number of setae present in the adult is reached at 3 rd Inst. (Fig. 43), with the addition of microchaeta a 1 a. After 3 rd Inst., chaetotacic changes are restricted to microchaetae length reduction and their progressive distancing from the macrochaetae (cf. Figs 41, 44). Adults have one microsensillum external to m 7, nine microchaetae, two acuminate mesochaetae (dots corresponding to m 7 and m 6 p, Fig. 44), and ten truncate macrochaetae. An interesting aspect of the development of this segment is that all macrochaetae on the anterior row external to a 2 move posteriorly into alignment with macrochaetae on the posterior row. The most dramatic case is the migration of a 6, which in the adult is located posterior to p 6.

Abdomen I. Setae absent at 1 st Inst. are a 7, m 1, m 7, and all setae on the posterior row, except p 5 and p 6 (Fig. 51). Only m 2–4 are truncate macrochaetae, although the sockets of a 1, m 6 and p 6 are large and could be confused with those of macrochaetae. All setae are ciliate, except the setula external to a 6. At 2 nd Inst. a new macrochaeta appears between m 4 and m 3 (Fig. 52). This seta appears almost directly posterior to a 3, suggesting that it is homologous to m 4 i present in some species of scaleless Orchesellinae , Entomobrya , Homidia and Himalanura ( Szeptycki 1979) . There are no changes in the structure of setae with respect to 1 st Inst., but a 5 and m 5 are more posterior and form a row with p 5 and p 6 (respectively), and as in other entomobryids, a 1 moves posterior to the pseudopore. The final set of setae present in the adult is completed at 3 rd Inst. with the addition of a 1 a (Fig. 53). At 4 th Inst. all microchaetae are smooth. In adults (Fig. 54) setae a 1 a, a 2, and a 3 do not reach the sockets of the macrochaetae, and a 6 and p 6 are aligned, forming a column with the lateral setula.

Abdomen II. The primary setae absent at 1 st Inst. are a 4, m 1, and p 1–3 (Fig. 60). All setae are ciliate, except as and se, and a 2 is inserted anterior to the pseudopore. At 2 nd Inst. macrochaeta m 3 e and microchaeta m 3 ea are added, whereas a 1, m 4, and p 4 are transformed into scales (Fig. 61). Seta m 7 moves anteriorly forming a row with a 7. No primary setae are lost after 2 nd Inst. Some of the fan-shaped setae around the botriothrica appear at 3 rd Inst. (triangles in Fig. 62), but their number varies from 2–4 around m 2 and 1–3 around a 5. I consider the fan-shaped setae posterior to m 2 homologous to a 2 p in Pseudosinella , although it appears at 3 rd Inst. in S. dowlingi and at 2 nd Inst. in P. alba ( Packard, 1873) . Based on their relative position, the fanshaped setae anterior to m 2 and posterior to a 5 may be re-diferentiated setae a 1 and m 4, respectivly. All fan setae of the botriothrical complexes present in the adult appear by 4 th Inst. (Fig. 63). By 4 th Inst. seta a 2 is consistently inserted posterior to the pseudopore. At 5 th Inst. seta a 2 is further displaced posteriorly. In adults all microchaetae, except p 6, are considerably shorter than at 1 st Inst. (Fig. 64).

Abdomen III. The primary setae absent at 1 st Inst. (Fig. 69) are a 4, m 1, p 1, and p 2. Setae as, d 2, and se are smooth; a 1, m 3, pm 6 and p 6 are ciliate macrochaetae; other setae are ciliate. Changes at 2 nd Inst. (Fig. 70) include the addition of setae m 3 ea, emp, and p 7 i; the transformation of a 1 into a microchaeta, which together with a 2 and p 3 are displaced close to m 2, becoming precursors of the fan setae of the botriothrical complex; and the relocation of m 4 closer to a 5. At 3 rd Inst. the chaetotaxy of area M is completed with the addition of two ciliate microchaetae (Fig. 71). The lateral botriothrical complex is unstable at this instar. All specimens add seta c 3 between pm 6 and emp, but three additional setae are asymmetrically present, in different combinations, in three of seven specimens examined. The chaetotaxy of the a 5 complex remains unstable at 4 th Inst.: three specimens have six setae, two have five setae, one has three setae on one side, and one has seven setae on one side. In addition, am 6 is a short truncate macrochaeta in two specimens. By 5 th Inst. the chaetotaxy is identical to the adult (Fig. 72): all setae in the botriothrical complexes are fan-shaped; am 6 is a macrochaeta; and p 7 i, and p 7 are ciliate, acuminate setae as long as m 3.

FIGURES 37 –41. 37– 40, Mesotergal chaetotaxy; 37, Seira brasiliana ; 38, S. desapercibida ; 39, S. blanca ; 40, S. annulicornis , redrawn from Szeptycki (1979); 41, S. dowlingi , 1 st instar metathorax, arrow points at supernumerary seta, asterisk identifies external microsensilla.

FIGURES 42–47. Dorsal metathoracic chaetotaxy; 42–44, Seira dowlingi ; 42, 2 nd instar; 43, 3 rd instar; 44, Adult; 45, S. blanca ; 46, S. desapercibida , setae m 6, m 7 and m 6 p are present but not shown; 47, S. brasiliana , m 6 p is present but not illustrated.

FIGURES 48 –54. 48– 50, Seira annulicornis ; 48–50, Metathorax, and alternative interpretations of internal setae homology; 51–54, S. dowlingi , chaetotaxy of first abdominal segment; 51, 1 st instar; 52, 2 nd instar; 53, 3 rd instar; 54, Adult, seta m 6 is present but not shown.

FIGURES 55 –60. 55– 59, Dorsal chaetotaxy of first abdomen; 55, Seira steinmetzi ; 56, S. blanca ; 57, S. brasiliana ; 58, S. desapercibida ; 59, S. annulicornis , redrawn from Szeptycki (1979); 60, S. dowlingi 1 st instar, dorsal chaetotaxy of second abdominal segment.

Abdomen IV. In addition to the regular primary and secondary setae, this segment has smooth, tapered, blunt, supplementary setae that often appear thin-walled. As in other segments, primary and secondary setae are structurally indistinguishable in the adult. The supplementary setae appear at 1 st Inst. and are, by definition, primary, but their position often varies between specimens and even between sides of the same specimen. The supplementary setae are restricted to the area between the pseudopore and column C, except for ps which is always associated with T 7. Supplementary setae are not labeled (except ps) and only their number is mentioned in the following description and discussion.

The chaetotaxy of S. dowlingi is arranged into 7 columns with varying number of setae per column (Fig. 74). Features deserving notice at 1 st Inst. are the presence of B 1 and the absence of A 4 and T 1. The primary macrochaetae correspond to B 4, B 5 and E 3. There are 10–11 supplementary setae. The fan-shaped setae associated with the botriothrical complexes and the posterior botriothricum are undifferentiated (Fig. 75). The setae added at 2 nd Inst. (Fig. 76) are A 3 a, A 4 s, C 1 p, T 1 s, T 1 p, D 2 a, D 3 p, E 2 p, E 4 p, E 4 p 2, F 1 p, F 3 p, three additional setae of uncertain homology postero-external to columns Fe, and five posterior setae. Primary setae B 6, C 1, F 2 and F 3 change into macrochaetae. The posterior botriothricum corresponds to seta D 3. The setae added at 3 rd Inst. (Fig. 77) are Ae 7 and Ee 10; A 3 becomes a macrochaeta, and some setae in the botriothrical complexes change into fans (triangles in Fig. 77). All setae added at 4 th Inst. belong to columns external to C, but those on E and F are variable and it is difficult to consistently ascertain their presence or absence in all specimens studied. Setae associated with the bothriotrichial complexes appearing at 4 th Inst. (drawing not included) are m, D 1 p, Pi, and Pe. Setae A 4 s, B 3, T 7, E 4, E 2, and Ee 10 are transformed into type two macrochaetae (i. e., acuminate instead of clubbed or truncate). The botriothrical complexes are completed at 5 th Inst. (Fig. 79); otherwise, this instar differs from the adult (Fig. 78) only in lacking setae A 2 a and in the number of macrochaetae in rows E, F, and Fe.

Figures

FIGURES 27 – 30. Adult dorsal head chaetotaxy; 27, Seira dowlingi; 28, S. desapercibida; 29, S. brasiliana; 30, S. blanca.

FIGURES 1 – 2. 1, Synthesis of adult dorsal head chaetotaxy in Seira blanca, S. brasiliana, S. desapercibida, S. dowlingi and S. steinmetzi; 2, Seta types present in Seira.

FIGURES 80 – 83. Dorsal chaetotaxy of fourth abdominal segment; 80, Seira blanca; 81, S. steinmetzi; 82, S. desapercibida; 83, S. brasiliana.