Corpora Cardiaca

The corpora cardiaca (singular: corpus cardiacum; from the Latin for "cardiac bodies," so named because of their intimate attachment to the cephalic wall of the dorsal vessel) are a pair of composite neurohemal and intrinsic endocrine organs located inside the retrocerebral complex immediately posterior to the supraesophageal brain of insects (Entomology; Nijhout, 1994). Positioned dorsal to the foregut (esophagus) and connected to the protocerebrum by paired nervi corporis cardiaci (NCC I, II, and III) as well as to the neighboring corpora allata (which synthesize juvenile hormone) via the nervi corporis allati, the corpora cardiaca serve as the functional analog of the vertebrate hypothalamo-pituitary axis (specifically combining the neurohemal posterior pituitary and the glandular anterior pituitary; Scharrer & Scharrer, 1944; Hartenstein, 2006). Histologically and developmentally, each corpus cardiacum is partitioned into two distinct zones: (1) a storage (neurohemal) lobe consisting of arborizing axon terminals from cerebral neurosecretory cells that store and exocytose brain hormones—including insulin-like peptides, diuretic hormones, corazonin, and in many orders prothoracicotropic hormone (PTTH)—directly into the circulating hemolymph; and (2) an intrinsic glandular lobe composed of neuroectodermally derived secretory cells (AKH cells) that synthesize and secrete adipokinetic hormone (AKH), the primary metabolic glucagon-like peptide that mobilizes trehalose and diacylglycerol from the fat body during sustained insect flight and starvation (Gäde et al., 1997; Van der Horst et al., 2001).

Dual Histological Compartments and Relationship to the Ring Gland

In generalized hemimetabolous insects such as Orthoptera (Schistocerca gregaria, Locusta migratoria), cockroaches (Blattodea), and Hemiptera, as well as adult Lepidoptera and Coleoptera, the retrocerebral endocrine complex sits just behind the brain flanking the hypocerebral ganglion of the stomatogastric nervous system and the dorsal aorta (Snodgrass, 1935; Nijhout, 1994):

1. Neurohemal Storage Lobe (Extrinsic Cerebral Terminals): Axons originating from the pars intercerebralis (PI) (median neurosecretory cells, MNCs) exit the posterior brain via the decussating nervus corporis cardiaci I (NCC I), while axons from the pars lateralis (PL) (lateral neurosecretory cells, LNCs) exit via the ipsilateral nervus corporis cardiaci II (NCC II), and tritocerebral/subesophageal axons enter via NCC III. Within the storage lobe of the corpus cardiacum, these axons branch into thousands of varicose neurohemal endings packed with 100–300 nm electron-dense neurosecretory granules abutting the fenestrated basal lamina of the dorsal aorta. Upon depolarization, voltage-gated Ca²⁺ influx triggers exocytosis of stored cerebral neuropeptides directly into the aortic hemolymph stream as it is pumped forward into the head by the dorsal vessel.
2. Intrinsic Glandular Lobe (Adipokinetic Hormone Secretory Cells): Differentiating during embryogenesis from invaginating placodes of the stomodeal (foregut) neuroectoderm via sine oculis and glass transcription factors, the intrinsic glandular lobe consists of densely packed, multipolar endocrine cells (intrinsic CC / AKH cells) bearing short processes enveloped by glial cells.
3. Condensation into Weismann's Ring (The Dipteran Ring Gland): In the larvae of cyclorrhaphous Diptera (such as Drosophila melanogaster, Musca domestica, and Calliphora), the three major retrocerebral and thoracic endocrine glands fuse into a single circular composite organ encircling the anterior aorta called Weismann's ring (the ring gland): the corpus allatum sits at the dorsal apex, the large steroidogenic prothoracic gland cells form the two lateral arms, and the corpora cardiaca form the ventral bridge where cerebral axons enter. During pupal holometabolism, the larval prothoracic gland degenerates while the corpora cardiaca and corpus allatum migrate posteriorly into the anterior prothorax above the cardia (proventriculus) of the adult fly.

Because winged insects achieve the highest mass-specific metabolic rates known in the animal kingdom—increasing their O₂ consumption 50- to 100-fold within seconds of takeoff—sustained insect flight requires rapid hormonal coordination between the corpora cardiaca, the fat body, and the thoracic flight muscles (Gäde et al., 1997; Van der Horst et al., 2001):

Dual-Phase Trehalose and Diacylglycerol Shuttling in Locusts

In long-distance migratory locusts (Locusta migratoria and Schistocerca gregaria, whose cambered tegmina and hindwings beat continuously for hours across hundreds of kilometers), flight metabolism proceeds in two hormonally regulated phases:

1. Carbohydrate Phase (First 15–30 Minutes of Flight) and AKH-I/II Release: During the initial minutes of flight, thoracic muscles oxidize circulating hemolymph trehalose (a non-reducing disaccharide of α-D-glucose). Octopaminergic fibers innervating the intrinsic glandular lobe of the corpora cardiaca—together with falling hemolymph trehalose sensed directly by ATP-sensitive K⁺ channels (K_ATP) and AMPK on the AKH cells—trigger exocytosis of Adipokinetic Hormones I, II, and III (primarily the decapeptide AKH-I, pGlu-Leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH₂).
2. Activation of the Fat Body AKH Receptor (G_q/G_s Cascade): AKH binds the seven-transmembrane AKH receptor (AKHR) on fat body trophocytes, activating both adenylyl cyclase (raising cAMP and activating PKA) and phospholipase C (generating IP₃ and releasing endoplasmic reticulum Ca²⁺). In the short term, Ca²⁺ and PKA activate glycogen phosphorylase to replenish hemolymph trehalose from fat body glycogen reserves.
3. Lipid Phase (Sustained Flight >30 Minutes) and the Lipophorin Shuttle: As flight continues beyond 20–30 minutes, PKA phosphorylates lipid-droplet perilipin (Lsd1) and activates triacylglycerol lipases, hydrolyzing stored triacylglycerols into 1,2-diacylglycerol (DAG). Unlike vertebrates (which transport free fatty acids bound to albumin), insects load hydrophobic DAG at the fat body surface onto reusable hemolymph high-density lipophorin (HDLp) particles together with exchangeable apolipophorin III (apoLp-III) molecules, converting HDLp into buoyant low-density lipophorin (LDLp). At the thoracic flight muscles, membrane-bound lipophorin lipase cleaves DAG into free fatty acids for mitochondrial β-oxidation, regenerating HDLp and free apoLp-III to cycle back to the fat body without requiring de novo apoprotein synthesis.

Proline Oxidation in Tsetse Flies and Beetles

In obligate hematophagous tsetse flies (Glossina morsitans, which house the vitamin-provisioning bacteriome endosymbiont Wigglesworthia) as well as scarab and Colorado potato beetles (Leptinotarsa decemlineata), the intrinsic AKH cells of the corpora cardiaca secrete hypertrehalosemic/hyperprolinemic AKH peptides that stimulate the fat body to synthesize the amino acid L-proline from lipid-derived acetyl-CoA and alanine. Delivered via the hemolymph to the thoracic flight muscles, proline undergoes partial oxidation in mitochondria to L-alanine (generating 2.5 moles of ATP per mole of proline without releasing NH₃), after which alanine returns to the fat body to be re-carboxylated and converted back into proline in a closed metabolic shuttle (Gäde et al., 1997).

Just as pancreatic insulin and glucagon form a counter-regulatory loop in mammals, cerebral Insulin-Like Peptides (ILPs) and Corpora Cardiaca AKH form a reciprocal metabolic feedback loop in insects (Kim & Rulifson, 2004): after a carbohydrate or blood meal, elevated circulating sugars and amino acids stimulate median brain neurosecretory cells to release ILPs through the neurohemal lobe of the corpora cardiaca while inhibiting intrinsic AKH cells; during starvation or flight, falling trehalose suppresses ILPs and disinhibits AKH secretion. In female Anopheles and Aedes mosquitoes, this corpora cardiaca ILP/AKH axis governs the partitioning of blood-meal lipids and amino acids into ovarian vitellogenesis and modulates the lipid resources available in the hemocoel for developing Plasmodium oocysts.

1. Scharrer, B., & Scharrer, E. (1944). Neurosecretion VI. A comparison between the intercerebralis-cardiacum-allatum system of the insects and the hypothalamo-hypophyseal system of the vertebrates. Biological Bulletin, 87(3), 242–251. https://doi.org/10.2307/1537959
2. Gäde, G., Hoffmann, K. H., & Spring, J. H. (1997). Hormonal regulation in insects: Facts, gaps, and future directions. Physiological Reviews, 77(4), 963–1032. https://doi.org/10.1152/physrev.1997.77.4.963
3. Van der Horst, D. J., Van Marrewijk, W. J., & Diederen, J. H. (2001). Adipokinetic hormones of insect: Release, signal transduction, and responses. International Review of Cytology, 211, 179–240. https://doi.org/10.1016/S0074-7696(01)11019-311019-3)
4. Kim, S. K., & Rulifson, E. J. (2004). Conserved mechanisms of glucose sensing and regulation by Drosophila corpora cardiaca cells. Nature, 431(7006), 316–320. https://doi.org/10.1038/nature02897
5. Hartenstein, V. (2006). The neuroendocrine system of invertebrates: A developmental and evolutionary perspective. Journal of Endocrinology, 190(3), 555–570. https://doi.org/10.1677/joe.1.06964