Transient use of hemolymph for hydraulic wing expansion in cicadas

Abstract

Insect wings must be flexible, light, and strong to allow dynamic behaviors such as flying, mating, and feeding. When winged insects eclose into adults, their wings unfold, actuated hydraulically by hemolymph. Flowing hemolymph in the wing is necessary for functioning and healthy wings, both as the wing forms and as an adult. Because this process recruits the circulatory system, we asked, how much hemolymph is pumped into wings, and what happens to the hemolymph afterwards? Using Brood X cicadas (Magicicada septendecim), we collected 200 cicada nymphs, observing wing transformation over 2 h. Using dissection, weighing, and imaging of wings at set time intervals, we found that within 40 min after emergence, wing pads morphed into adult wings and total wing mass increased to ~ 16% of body mass. Thus, a significant amount of hemolymph is diverted from body to wings to effectuate expansion. After full expansion, in the ~ 80 min after, the mass of the wings decreased precipitously. In fact, the final adult wing is lighter than the initial folded wing pad, a surprising result. These results demonstrate that cicadas not only pump hemolymph into the wings, they then pump it out, producing a strong yet lightweight wing.

Figure 1

Wing expansion in cicadas. (A) Prior to wing expansion, juvenile cicada nymphs crawl up a vertical surface, quiesce, and initiate eclosion, the process of adult emergence (left). Sclerotization and melanization occur over the next 24 h (right). (B) To pull out its wings, Magicicada septendecim split the exoskeleton of the thorax and wriggle out. Four wings are freed during this process as the cicada continues to bend backwards. At about 40 min, the wings are fully expanded, and at 50 min they are folded over the abdomen. (C) Approximately 800 wings were dissected, weighed, and imaged in 5-min intervals during expansion from wings out (time t = 0 to 120 min). (i) Mass was measured immediately after dissection. Wing mass increased up until 40 min, then mass decreased over the next 2 h. (ii) Span (i.e., wing length) plateaued at 40 min, measured in FIJI post-experiment (See Fig. 3 for chord and area). (iii) At the end of the expansion phase, total wing mass accounts for ~16% of the insect’s mass (relative to body mass). (iv) Body mass over expansion. (D) Images of dissected fore- and hind-wings throughout expansion. (E) Log of the total wing mass versus log of span (i–iii) compared predicted isometric versus actual scaling relationships, with a least-squares regression fit and a 95% confidence interval (See Fig. 3 for log of chord and area relationships).

Figure 2

Eclosion and wing expansion on a “cicada wall”. An 8-foot wall of insect netting was mounted in a walled-in porch. Nymphs were placed on the wall and allowed to crawl freely until they settled to initiate emergence. (A) Nymphs have climbed up the mesh wall. (B) Cicadas emerging on the wall with tape labels indicating timing intervals. (C) As more cicadas emerge, more colors of tape are used to differentiate expansion at different times. (D) Side-view of the emergence wall. (E) A cicada in the backwards-bend phase of eclosion. When it flips forward to extract the abdomen, this is time "zero". (F) Due to the quick pace of emergence, many cicadas were not labelled. Cicadas then eclosed and were returned to their site of capture.

Figure 3

Body mass, area, and chord relationships. (A) Body mass trends for female (yellow) and male (gray) cicadas from t = 0 to 120 min. Data were pooled in the main study as there were no effects of sex on the statistical models. (B) Masses for body, left forewings, and left hindwings, for female (yellow) and male (gray) cicadas. Only the left side is represented, as there were no significant differences between the left and right wing mass trends. (C) Area and chord represented for all wings, both left and right; however, there were no significant differences between left and right measurements. (D) Scaling patterns of log total wing chord and area relative to log of wing span. These relationships followed predictions of geometric isometry.