Tanning of the tarsal and mandibular cuticle in adult Anax imperator (Insecta: Odonata) during the emergence sequence

Abstract

The arthropod cuticle offers strength, protection, and lightweight. Due to its limit in expandability, arthropods have to moult periodically to grow. While moulting is beneficial in terms of parasite or toxin control, growth and adaptation to environmental conditions, it costs energy and leaves the soft animal's body vulnerable to injuries and desiccation directly after ecdysis. To investigate the temporal change in sclerotization and pigmentation during and after ecdysis, we combined macrophotography, confocal laser scanning microscopy, scanning electron microscopy and histological sectioning. We analysed the tarsal and mandibular cuticle of the blue emperor dragonfly to compare the progress of tanning for structures that are functionally involved during emergence (tarsus/tarsal claws) with structures whose functionality is required much later (mandibles). Our results show that: (i) the tanning of the tarsal and mandibular cuticle increases during emergence; (ii) the tarsal cuticle tans faster than the mandibular cuticle; (iii) the mandibles tan faster on the aboral than on the oral side; and (iv) both the exo- and the endocuticle are tanned. The change in the cuticle composition of the tarsal and mandibular cuticle reflects the demand for higher mechanical stability of these body parts when holding on to the substrate during emergence and during first walking or hunting attempts.

Keywords: Anisoptera; cuticle; insect; leg; mouthpart; sclerotization.

Figure 1.

Emergence sequence and subsequent stages (emergence sequence + 12, 24, 72 h and 7 d) of specimens used for this study. (a) Head and thorax released from exuvium (st1). (b) Resting period (30 min after start of emergence sequence) (st2). (c) Body flips over (1.5 h after the start of emergence sequence) (st3). (d) Abdomen withdrawn from exuvium (2 h after the start of emergence sequence) (st4). (e) After expansion of wings (2.5 h after the start of emergence sequence) (st5). (f) Maiden flight (3 h after the start of emergence sequence) (st6). (g) 12 h after the maiden flight (st7); 24 h after the maiden flight (st8); 72 h after the maiden flight (st9); 7 days after the maiden flight (st10). For simplicity, the abbreviations of the individual stages mentioned here are used in the text to characterize the developmental state of the samples.

Figure 2.

Photographic overview of mandible and tarsus of A. imperator. (a) Mandible from oral view. (b) Mandible from aboral view. (c) Tarsus from lateral view. aar, anterior acetabular ridge; adr, anterior dorsal ridge; air, acetabular–incisivi ridge; amp, aboral mandibular pad; cl, tarsal claws; cir, condylar–incisivi ridge; cmr, circum-mandibular ridge; dmp, dorsal mandibular pad; inc, incisivi; mmr, median molar ridge; mol, mola; pcr, posterior condylar ridge; pdr, posterior dorsal ridge; pma, posterior mandibular articulation; set, setae; t, tarsomeres.

Figure 3.

Dome photographs (a–f) and CLSM maximum intensity projections (g–l) of the tarsus of A. imperator during and after the emergence sequence. (a,g) Head and thorax released from exuvium (st1). (b,h) Abdomen withdrawn from exuvium (st4). (c,i) After expansion of wings (st5). (d,j) Maiden flight (st6). (e,k) 12 h after maiden flight (st7). (f,l) 7 days after maiden flight (st10). cl, tarsal claws; set, setae; t, tarsomeres. Scale bars, 300 µm.

Figure 4.

LM images. Cross-sections of the tarsal claw cuticle stained with toluidine blue (left; a1–f1) and Cason's triple stain (right; a2–f2). (a1, a2) st1. (b1, b2) st4. (c1, c2) st5. (d1, d2) st6. (e1, e2) st7. (f1, f2) st10. ed, epidermis; endo, endocuticle; exo, exocuticle; gz, growing zone. Scale bars, 40 µm.

Figure 5.

SEM images, tarsal claw fracture in A. imperator 10 days after the maiden flight. (a) Overview sectional plane. (b) Schematic localization of the fracture within the tarsal claw. (c) SEM image, cross section, overview of the cuticle layering in the tarsal claw. exo, exocuticle; endo, endocuticle.

Figure 6.

Dome photographs (a–f; m–r) and CLSM maximum intensity projections (g–l; s–x) of the mandible of A. imperator during and after the emergence sequence. (a,g,m,s) Head and thorax released from exuvium (st1). (b,h,n,t) Abdomen withdrawn from exuvium (st4). (c,i,o,u) After expansion of wings (st5). (d,j,p,v) Maiden flight (st6). (e,k,q,w) 12 h after maiden flight (st7). (f,l,r,x) 7 days after maiden flight (st10). (a–l) Aboral view. (m–x) Oral view. aar, anterior acetabular ridge; amp, aboral mandibular pad; dmp, dorsal mandibular pad; inc, incisivi; mol, mola; set, setae.

Figure 7.

LM images. Cross-sections of the mandibular cuticle stained with toluidine blue (left; a1–f1) and Cason's triple stain (right; a2–f2). (a1, a2) st1. (b1, b2) st4. (c1, c2) st5. (d1, d2) st6. (e1, e2) st7. (f1, f2) st10. ed, epidermis; endo, endocuticle; exo, exocuticle; gz, growing zone; (1) incisivi; (2) condylar incisivi ridge (cir; oral view); (3) aboral mandibular pad (amp; oral view); (4) dorsal mandibular pad (dmp; aboral view). Scale bars, 40 µm.

Figure 8.

SEM images, mandible fracture in A. imperator 10 days after the maiden flight. (a) Overview sectional plane. (b) SEM image, longitudinal fracture, overview of the cuticle layering in the molae. (c–e) SEM images, magnification of specific subsections from the longitudinal section of (b). exo, exocuticle; endo, endocuticle.