Megalothorax perspicillum, Schneider & D'Haese, 2013

Morphogenesis of M. perspicillum Schneider & D’Haese, 2013 View in CoL

External body morphology of juvenile stages. The first juvenile stage ( Fig. 1A View FIGURE 1 ) has 12 chaetae on the thorax (vs 22 chaetae at the final stage) and 13 chaetae on the abdomen (vs 18 chaetae at the final stage). Chaetotaxy on the a- and p-rows on the second and third thoracic segments is very different from that of the adult. The α- or β-rows of the abdomen are represented by a single chaeta (these two rows together consist of five chaetae at the final stage); the remaining rows change little: number of chaetae here are close to that at the final developmental stage. After the first molt (second stage), chaetae appear on the p -row of the second and third thoracic segments; wrc-6 and wrc-7, and β-row chaetae also appear on abdomen ( Fig. 1B View FIGURE 1 ). After the second molt (adult stage), the chaetotaxy of the thorax and abdomen completely corresponds to the adult sexually mature stage ( Fig. 1C View FIGURE 1 ). The integumentary canals are developed at all stages. Sensillae of the thorax and abdomen (s1, s2, and s3) are already present at the first stage. Sensilla s3 is much larger than s2 at juvenile stages of development ( Figs 1A–B View FIGURE 1 ); in adult specimens, these sensillae are of the same size ( Fig. 1C View FIGURE 1 ). The internal sensillae of the sensory fields are spherical, developed at all stages ( Figs 1A–C View FIGURE 1 ). The chaetotaxy of the thorax and abdomen at different stages of development is presented in Table 1.

Head morphology of juvenile stages. At the first developmental stage, the chaetotaxy of the occipital part and the forehead of the head completely corresponds to that in adult sexually mature stage ( Fig. 2A View FIGURE 2 ). Only the chaeta rows in the middle part of the head undergo changes during morphogenesis. Chaeta a0 is already present at the first developmental stage ( Fig. 2A View FIGURE 2 ). The series of ia. - chaetae is absent at the first stage and partially appears during ontogenesis already at the second stage ( Fig. 2B View FIGURE 2 ). After the second molt, the chaetotaxy of the head completely corresponds to the adult sexually mature stage ( Fig. 2C View FIGURE 2 ). The chaetotaxy of the head at different stages of development is presented in Table 2.

Antennae morphology of juvenile stages. In general, the antennal chaetome at the first stage is almost identical to the adult, except for a few chaetae on Ant III and Ant IV and the sensilla s ( Figs 3A–C View FIGURE 3 ). At the second stage, only one chaeta appears on Ant III. After the second molt, the chaetotaxy of all antennal segments becomes similar to that of the adult. The number of chaetae for each segment at each stage are summarized in Table 3.

Leg morphology of juvenile stages. At the first developmental stage, there is no chaeta on the coxa of leg I (vs. 1 chaeta at the second and final stage) and only one chaeta on the trochanter (vs. 3 chaetae at the second and final stage). Chaetotaxy on the leg I corresponds to that of adult sexually mature individual already at the second developmental stage ( Fig. 4A–C View FIGURE 4 ). On the leg II, changes of chaetotaxy after the first molt occur only on trochanter ( Fig. 5A–C View FIGURE 5 ). After the second molt, one chaeta appears on coxa, and one chaeta, on femur. Leg III undergoes the greatest changes during morphogenesis ( Fig. 6A–C View FIGURE 6 ). Quantity of chaetae for each segment at each stage are summarized in Table 4.

Chaetotaxy of furcula is identical to chaetotaxy of an adult individual already at the first developmental stage.

Morphogenesis of M. bokovae Panina & Potapov, 2025 ( in: Panina et al. 2025)

External body morphology of juvenile stages. During ontogenesis, the chaetotaxy development patterns on the thorax and abdomen is similar to M. perspicillum . The first stage ( Fig. 7A View FIGURE 7 ) has 12 chaetae on the thorax (vs. 22 chaetae in the final stage) and 14 chaetae on the abdomen (vs. 19 chaetae in the final stage). Chaetotaxy on the a- and p-rows on the second and third thoracic segments is very different from the adult stage. In addition, there is no wrc-4 chaeta (vs. present in M. perspicillum ), wrc-6 and a thickened wrc-7 (unique feature of the species) are present on the third thoracic segment. The α- or β-rows of the abdomen are represented by a pair of chaetae; these two rows together consist of seven chaetae at the final stage. The remaining rows change little: the chaetae number in these rows are close to the final developmental stage. At the second stage, wrc-4, wrc- 6 in the p-row and wrc- 7 in the αrow chaetae appear in the p-row of the second and third thoracic segments ( Fig. 7B View FIGURE 7 ). The emerging wrc-7 chaeta is thickened, similar to a candle inserted into a candlestick, similar to that at the adult sexually mature stage ( Fig. 14D View FIGURE 14 ). Chaetae of the α- and β-rows of the abdomen (chaeta α2 and β1) also appear. After the second molt, the chaetotaxy of the thorax and abdomen corresponds completely to the adult sexually mature stage ( Fig. 7C View FIGURE 7 ). Sensilla of the thorax and abdomen, s1 and s2, are already present at the first developmental stage ( Fig. 7A View FIGURE 7 ; 14A–D View FIGURE 14 ). Sensillum s3 is absent, at the adult stage as well. The internal sensilla of the sensory fields are flame-shaped, developed at all stages ( Figs 7 View FIGURE 7 A-C; 14A–D). Secondary granulation is already developed at the first stage of development ( Fig. 13 View FIGURE 13 ). The chaetotaxy of the thorax and abdomen at different stages of development is presented in Table 1.

Head morphology of juvenile stages. The head morphogenesis of M. bokovae is similar to that of M. perspicillum . As early as at the first developmental stage, chaetotaxy of the occipital part and forehead of the head completely corresponds to that at the adult sexually mature stage ( Fig. 8A View FIGURE 8 ). Only the chaetae rows of the middle part of the head undergo changes during morphogenesis. Chaeta a0 is absent at the first developmental stage, in adults as well ( Fig. 8A View FIGURE 8 ). Also, some of chaetae ia. are absent at the first stage, they partially appear during ontogenesis at the second stage ( Fig. 8B View FIGURE 8 ). After the second molt, the head chaetotaxy completely corresponds to the adult sexually mature stage ( Fig. 8C View FIGURE 8 ). Another unique character of M. bokovae , cuticular outgrowth, appears as early as at the first developmental stage, although it has a different shape. In individuals that have just hatched from the egg, the “nose” has a slightly pointed shape, while this structure has the shape of a small spherical cuticular outgrowth at the second developmental stage and adults ( Fig. 8A–C View FIGURE 8 ; 14E View FIGURE 14 ). The chaetotaxy of the head at different stages of development is presented in Table 2.

ontogenesis are marked in red.

Antennae morphology of juvenile stages. In general, the antennal chaetome at the first stage is almost identical to that of the adult, except for a few chaetae on Ant III and Ant IV and the sensilla s ( Fig. 9A–C View FIGURE 9 ). At the second stage, only one chaeta appears on Ant. III (a6) and two sensilla on Ant. IV. After the second molt, the chaetotaxy of all antennal segments becomes identical to that of the adult. The number of chaetae for each segment at each stage are summarized in Table 3.

Leg morphology of juvenile stages. At the first developmental stage, chaetotaxy on the leg I is identical to that at the second developmental stage ( Figs 10A–B View FIGURE 10 ). New chaetae on the leg I appear only at the final developmental stage on the coxa, femur, trochanter, and tibiotarsus. On the leg II, chaetotaxy undergoes changes already after the first molt, when three new chaetae appear on the tibiotarsus ( Fig. 11A–C View FIGURE 11 ).At the second stage, chaetae appear on the coxa, femur, and trochanter. Chaetotaxy on leg III changes the most. At the second developmental stage, one chaeta appears on the coxa and femur each, and three chaetae, on the trochanter ( Fig. 12A–C View FIGURE 12 ). After the second molt, the chaetotaxy of the leg III is completely identical to the adult. The number of chaetae for each segment at each stage are summarized in Table 4.

Chaetotaxy of furca is the same to that of adult individual as early as at the first developmental stage.