Dorsal Diaphragm (Entomology)

The dorsal diaphragm is a horizontal, fenestrated fibromuscular septum stretching across the dorsal roof of the abdominal cavity (hemocoel) in insects that suspends the tubular abdominal heart (dorsal vessel) from the lateral tergal body wall and partitions the upper abdomen into a dorsal pericardial sinus and a central perivisceral sinus. Constructed from metameric pairs of fan-shaped striated alary muscles (musculi alares) woven into an extracellular matrix of collagen IV (Viking) and fibrillin-like elastic microfibrils (Pericardin), the dorsal diaphragm performs three integrated physiological functions across Hexapoda (Glenn, King, & Hillyer, 2010; Hillyer & Pass, 2020): (1) mechanically anchoring the heart along the dorsal midline and generating the outward diastolic elastic recoil that re-expands the heart lumen after each systolic contraction to aspirate hemolymph through the cardiac ostia; (2) serving as the structural scaffold for rows of pericardial nephrocytes, podocyte-like filtration cells that endocytose and detoxify hemolymph proteins and metabolic wastes before fluid enters the heart; and (3) hosting the periostial hemocyte immune aggregates that phagocytose and melanize blood-borne bacteria, arboviruses, and malaria parasites (Plasmodium) in disease vectors such as Anopheles gambiae (King & Hillyer, 2012).

Boundary of the Dorsal Pericardial Sinus

Within the open circulatory system of an insect abdomen, the dorsal diaphragm forms the floor of the pericardial sinus—the specialized dorsal fluid channel running from abdominal segment A1/A2 to A7/A8 that surrounds the pulsatile abdominal heart (dorsal vessel), whereas the ventral diaphragm forms the ceiling of the ventral perineural sinus around the nerve cord (Hillyer & Pass, 2020).

Because the connective-tissue membrane of the dorsal diaphragm is perforated by numerous lateral and intersegmental fenestrations (openings), hemolymph from the central perivisceral sinus (bathing the midgut, Malpighian tubules, and fat body) is drawn upward across the dorsal diaphragm into the pericardial sinus whenever the heart expands during diastole. In large insects such as grasshoppers (Orthoptera), cockroaches (Blattodea), and hawkmoths (Lepidoptera), the dorsal diaphragm forms an almost continuous, gently arched muscular sheet beneath the heart; when its alary muscle fibers contract rhythmically in slow undulating waves, they depress the diaphragm roof downward, actively expanding the volume of the pericardial sinus and aspirating perivisceral hemolymph upward toward the heart ostia.

Paired Fan-Shaped Alary Muscles (Musculi Alares)

The muscular skeleton of the dorsal diaphragm consists of segmental pairs of alary muscles (typically 6 to 8 pairs in the abdomen, corresponding to segments A1/A2 through A7/A8):

1. Lateral Tergal Origin: Each alary muscle originates as a narrow, focal tendon attached to the intersegmental ridge of the lateral tergal insect cuticle via specialized microtubule-rich epidermal attachment cells (tendon cells expressing Stripe and βPS-integrin).
2. Medial Delta-Shaped Myofibrillar Fan: From its lateral focal origin, each alary muscle diverges medially into a broad triangular delta of striated myofibrils that splits into dorsal and ventral fascicles. These myofibrils arch beneath and around the lateral walls of the tubular heart, particularly flanking the segmental incurrent ostia (valved inflow slits), and interdigitate across the dorsal midline with the contralateral alary muscle.
3. The Pericardin Elastic Network: Rather than inserting via rigid focal myotendinous junctions directly onto the cardiomyocytes, the medial ends of the alary muscles branch into an extensive three-dimensional anastomosing meshwork of Pericardin (a specialized Drosophila and mosquito collagen IV-like glycoprotein) and fibrillin-like oxytalan fibers (Chartier et al., 2002; Wilmes et al., 2023). Genetic ablation of pericardin or surgical laser severing of the alary muscles causes the abdominal heart to detach from the body wall, collapse inward during systole, and lose its diastolic stroke volume.

Because the insect heart lacks stiff cartilaginous rings or high internal arterial blood pressure to pop its lumen back open after each contraction, the dorsal diaphragm and alary muscles act as the antagonistic diastolic expander of the heart (Glenn, King, & Hillyer, 2010; League et al., 2015):

• During Cardiac Systole (Contraction): The circular and helical striated cardiomyocytes of the dorsal vessel contract in a peristaltic wave, constricting the heart lumen and forcing the inward-pointing lips of the incurrent ostia shut. As the outer diameter of the heart tube shrinks by 30% to 50%, it pulls inward against the attached alary muscles and Pericardin elastic fibers of the dorsal diaphragm, storing elastic strain energy in the stretched alary-pericardin spring.
• During Cardiac Diastole (Relaxation): As soon as the circular cardiomyocytes relax, the resting tension of the alary muscles and the elastic recoil of the Pericardin meshwork pull the lateral walls of the heart tube outward toward the body wall. This outward traction rapidly expands the heart lumen, generating a brief sub-atmospheric (negative) pressure pulse inside the heart chamber relative to the pericardial sinus. Under this pressure differential, hemolymph from the pericardial sinus pushes open the passive one-way valve flaps of the incurrent ostia and floods into the heart lumen to initiate the next stroke.

In Drosophila and Anopheles gambiae, the alary muscles themselves behave primarily as tonic viscoelastic suspensory springs during normal resting heartbeats, maintaining baseline tension on the heart wall while contracting phasically during abdominal peristalsis or neurohormonal stimulation by crustacean cardioactive peptide (CCAP), serotonin, and proctolin.

Woven directly into the collagenous web of the dorsal diaphragm and resting atop the alary muscle bundles flanking the heart are rows of large, often binucleate cells termed pericardial cells or pericardial nephrocytes (Weavers et al., 2009; Tutor et al., 2014).

In a landmark comparative discovery published in Nature in 2009, Helen Weavers, Silvia Prieto-Sánchez, Franziska Grawe, and Helen Skaer proved that insect pericardial nephrocytes are the direct evolutionary and molecular homologs of vertebrate renal glomerular podocytes:

1. The Nephrocyte Slit Diaphragm: The plasma membrane of each pericardial nephrocyte invaginates into thousands of narrow foot processes (labyrinthine channels) bridged by a 40-nm slit diaphragm constructed from the immunoglobulin-superfamily transmembrane proteins Sticks and stones (Sns) (the insect ortholog of human Nephrin, mutated in congenital nephrotic syndrome) and Dumbfounded/Kirre (the ortholog of NEPH1), anchored intracellularly by Polychaetoid (ZO-1) and Mec-2 (Podocin).
2. Two-Stage Ultrafiltration and Cubilin/Amnionless Endocytosis: Because the insect open circulatory system operates at low hydrostatic pressure (<1 kPa), nephrocytes cannot filter blood by transcapillary pressure like a vertebrate glomerulus. Instead, as the dorsal diaphragm channels hemolymph through the pericardial sinus toward the heart ostia, the nephrocyte basement membrane and Nephrin/NEPH1 slit diaphragm act as a charge- and size-selective molecular sieve (~50–70 kDa cutoff) that admits small toxins, worn-out hemolymph peptides, and lysozyme into the labyrinthine channels, where Megalin, Cubilin, and Amnionless scavenger receptors rapidly clear them via clathrin-mediated endocytosis and lysosomal degradation!

In hematophagous disease vectors such as the malaria mosquito Anopheles gambiae and the yellow fever vector Aedes aegypti, the dorsal diaphragm serves as the structural platform for the periostial immune system discovered by Julián F. Hillyer and colleagues (Glenn et al., 2010; King & Hillyer, 2012; Sigle & Hillyer, 2016).

Because every drop of hemolymph entering the mosquito heart must funnel through the six pairs of abdominal ostia in segments A2 through A7, the Pericardin fibers of the dorsal diaphragm immediately flanking each ostium experience the highest fluid shear rates in the hemocoel. Specialized phagocytic granulocytes termed periostial hemocytes bind selectively to these alary-muscle Pericardin fibers outside each ostial portal. Within minutes of a bacterial infection, viral dissemination (dengue virus), or Plasmodium sporozoite release from midgut oocysts into the hemolymph, additional circulating hemocytes are recruited via the Janus kinase/signal transducer and activator of transcription (JAK/STAT) and Eater/Nimrod B2 scavenger-receptor pathways to the dorsal diaphragm, doubling the size of the periostial aggregates and destroying circulating pathogens before they can reach the mosquito salivary glands.

How do the alary muscles survive complete metamorphosis when larval abdominal muscles dissolve?

During pupal holometabolism in Drosophila and mosquitoes, most larval body-wall muscles undergo programmed histolysis. However, the anterior pairs of larval alary muscles survive histolysis or undergo remarkable transdifferentiation: as discovered by Cédric Bataillé et al. (2020), specific anterior alary muscles in Drosophila dedifferentiate during metamorphosis and remodel into the longitudinal ventral muscle fibers that ensheathe the adult anterior heart, while the remaining posterior alary pairs re-anchor to the adult tergal cuticle to suspend the adult dorsal vessel.

Why are pericardial nephrocytes positioned specifically on the dorsal diaphragm rather than scattered randomly in the fat body?

Positioning pericardial nephrocytes directly on the alary muscles of the dorsal diaphragm inside the pericardial sinus places them in the mandatory hydrodynamic inflow path of the heart. Because all hemolymph in the insect body must pass across the fenestrations of the dorsal diaphragm into the pericardial sinus before entering the cardiac ostia, anchoring nephrocytes along the alary muscle fibers ensures that 100% of the circulating hemolymph volume is continuously filtered across the nephrocyte slit diaphragms without requiring a closed vascular network.

1. Weavers, H., Prieto-Sánchez, S., Grawe, F., Garcia-Lopez, A., Artero, R., Wilsch-Bräuninger, M., et al. (2009). "The insect nephrocyte is a podocyte-like cell with a filtration slit diaphragm." Nature, 457(7227), 322–326. https://doi.org/10.1038/nature07526
2. Glenn, J. D., King, J. G., & Hillyer, J. F. (2010). "Structural mechanics of the mosquito heart and its function in bidirectional hemolymph transport." Journal of Experimental Biology, 213(4), 541–550. https://doi.org/10.1242/jeb.038638
3. King, J. G., & Hillyer, J. F. (2012). "Infection-induced interaction between the mosquito circulatory and immune systems." PLOS Pathogens, 8(11), e1003058. https://doi.org/10.1371/journal.ppat.1003058
4. League, L. A., Baxter, L. L., Wolfner, M. F., & Hillyer, J. F. (2015). "Structural and functional characterization of the ventral diaphragm and alary muscles of the mosquito Anopheles gambiae." Journal of Experimental Biology, 218(3), 370–379. https://doi.org/10.1242/jeb.114058
5. Hillyer, J. F., & Pass, G. (2020). "The insect circulatory system: Structure, function, and evolution." Annual Review of Entomology, 65, 121–143. https://doi.org/10.1146/annurev-ento-011019-025003