Hemiptera

Hemiptera (from the Ancient Greek hemi, "half," and pteron, "wing," coined by Carl Linnaeus in 1758) is the order of insects (Insecta, Arthropoda) that comprises all true bugs (Heteroptera), cicadas, leafhoppers, treehoppers, spittlebugs, and planthoppers (Auchenorrhyncha), and aphids, whiteflies, psyllids, and scale insects (Sternorrhyncha). With more than 100,000 described extant species across ~150 families, Hemiptera is the fifth-largest order of insects overall (after Coleoptera, Lepidoptera, Diptera, and Hymenoptera) and the largest non-holometabolous (hemimetabolous / incomplete-metamorphosis) insect order on Earth—representing the sister clade to Thysanoptera (thrips) within the paraneopteran superorder Condylognatha.

In everyday English, people casually call any insect or spider a "bug," but to an entomologist, only a member of the order Hemiptera (and strictly the suborder Heteroptera) is a "true bug"!

Regardless of whether a hemipteran feeds on plant phloem sap (aphids, whiteflies), xylem water (cicadas, spittlebugs), seeds (stink bugs), insect hemolymph (assassin bugs, giant water bugs), or vertebrate blood (bed bugs Cimex lectularius and Chagas-disease-transmitting kissing bugs Triatoma / Rhodnius), every member of Order Hemiptera shares a unique modification of the ancestral insect mouthparts—the piercing–sucking beak (rostrum):

1. The Segmented Labial Sheath and Interlocking Stylet Fascicle: The lower lip (labium) is modified into a jointed, grooved external protective sheath (rostrum) that folds backward beneath the head and thorax when at rest. Inside the labial groove sits the actual piercing needle—a slender, flexible bundle (stylet fascicle, often only 2–10 μm in diameter!) formed by four modified mouthparts:
   • An outer pair of serrated mandibular stylets that saw and anchor into plant cell walls or animal skin; and
   • An inner pair of maxillary stylets that lock together along longitudinal tongue-and-groove ridges to form two separate, parallel microscopic tubes inside a single needle:
   • A dorsal Food Canal, through which liquid sap, liquefied tissue, or blood is sucked upward by a muscular cibarial pump in the head; and
   • A ventral Salivary Canal, through which the insect pumps gel saliva (forming a sealing sheath in plant-feeders), pectinases, neurotoxins, anticoagulants (nitrophorins in Rhodnius), or plant-pathogenic viruses and phytoplasmas directly into the host!
2. Filter Chamber of the Midgut (Bypassing Sugar Osmotic Overload): Because plant phloem sap is excessively rich in sucrose (0.3–1.0 M) but poor in amino acids, and xylem sap is >99.8% pure water with trace minerals, sap-feeding hemipterans must drink 10 to 1,000 times their own body weight in fluid every day just to get enough nitrogen! To avoid bursting or diluting their hemolymph, cicadas, leafhoppers, whiteflies, and aphids loop the anterior midgut directly against the posterior midgut/hindgut inside a membranous wrap—the filter chamber—which shunts excess water and simple sugars straight to the rectum for immediate excretion as sugary "honeydew" (farmed by mutualistic ants and causing agricultural sooty mold!) while concentrating dietary amino acids in the midgut loop.

Historically, taxonomists split the order into two groups: "Heteroptera" (true bugs) and "Homoptera" (cicadas, hoppers, aphids, and scales). Molecular phylogenomics (Cryan & Urban, 2012; Johnson et al., 2018) proved that "Homoptera" is paraphyletic, dividing Hemiptera into four monophyletic suborders:

How can an aphid, psyllid, cicada, or scale insect grow and reproduce on a diet of 100% plant phloem sap or xylem water—which lacks 9 to 10 of the essential amino acids required to build animal proteins?

• Beginning over 160–280 million years ago, sap-feeding hemipterans evolved a specialized abdominal organ made of giant polyploid cells called bacteriocytes (forming the bacteriome), which house strictly vertically (maternally) transmitted intracellular bacterial endosymbionts that act like "amino-acid-synthesizing organelles" (Nancy A. Moran, Paul Baumann, and John P. McCutcheon):
  • Aphids + Buchnera aphidicola: Every pea aphid (Acyrthosiphon pisum) houses millones of γ-proteobacterial Buchnera aphidicola cells inside its bacteriocytes, which dedicate their shrunken ~640-kb genome to overproducing tryptophan, phenylalanine, leucine, and other essential amino acids for the aphid host!
  • Psyllids + Carsonella ruddii (160 kb) and Cicadas + Hodgkinia / Sulcia: Through hundreds of millions of years of maternal bottlenecking and Muller's ratchet, hemipteran endosymbionts suffered the most extreme genome reduction of any cellular organisms on Earth: the psyllid symbiont Carsonella ruddii has a circular genome of just 159,662 base pairs (160 kb, encoding only 182 proteins!) (Nakabachi et al., 2006), and the leafhopper symbiont Nasuia deltocephalinicola has the smallest known bacterial genome at just 112 kilobases (137 genes) (Bennett & Moran, 2013)—smaller than a plant chloroplast genome!
  • Mealybugs (Planococcus citri): A Bacterium Living Inside Another Bacterium Inside an Insect! In 2001 and 2011, abigail D. von Dohlen, John P. McCutcheon, and Nancy A. Moran discovered a three-tiered Russian-doll symbiosis in citrus mealybugs: the insect's bacteriocytes house the β-proteobacterium Tremblaya princeps (genome 139 kb, having lost even its own aminoacyl-tRNA synthetases!), and living inside the cytoplasm of Tremblaya is a second γ-proteobacterium, Moranella endobia, which collaborates step-by-step with Tremblaya and horizontal bacterial genes transferred to the insect nucleus (Husnik et al., 2013) to synthesize phenylalanine and peptidoglycan!

Why Do Periodical Cicadas (Magicicada) Emerge Only Every 13 or 17 Years (Prime-Number Life Cycles)?

In eastern North America, seven species of periodical cicadas (Magicicada septendecim, M. cassini, M. septendecula [17-year] and M. tredecim, M. neotredecim, M. tredecassini, M. tredecula [13-year]) spend 13 or 17 years underground as root-xylem-feeding nymphs (counting years via annual host-tree seasonal xylem amino-acid cycles; Karban et al., 2000), before emerging synchronously within a two-week window at densities of up to 1.5 million cicadas per acre! Why 13 and 17—both prime numbers—rather than 12, 14, 15, 16, or 18 years?

1. Predator Satiation: By emerging all at once in billions after >13 years underground, no bird, mammal, or fish predator population can maintain a 13- or 17-year life cycle tracking them; predators gorge until physically full within days (predator satiation), leaving >95% of cicadas unharmed to mate (except those infected by the behavioral-hijacking zombie fungus Massospora cicadina, which replaces the cicada's abdomen with a spore plug and doses the living insect with cathinone amphetamine to induce hypersexual wing-flicking transmission; Boyce et al., 2019!).
2. Prime-Number Avoidance of Multi-Year Predator/Parasitoid Harmonics and Sub-Brood Hybridization: As modeled by Stephen Jay Gould (1977), Glenn Webb (2001), and Teiji Sota et al. (2013), prime-number cycles (13 and 17) minimize coincidental resonance with 2-, 3-, 4-, or 5-year predator population cycles, and only co-emerge with each other once every 13 × 17 = 221 years (as occurred in May 2024 when 13-year Brood XIX and 17-year Brood XIII co-emerged across Illinois!), preventing intermediate hybrid cycle lengths from breaking up the synchronous predator-satiation shield!

1. Nakabachi, A., Yamashita, A., Toh, H., Ishikawa, H., Dunbar, H. E., Moran, N. A., & Hattori, M. (2006). The 160-kilobase genome of the bacterial endosymbiont Carsonella. Science, 314(5797), 267. doi:10.1126/science.1134196
2. McCutcheon, J. P., & Moran, N. A. (2012). Extreme genome reduction in symbiotic bacteria. Nature Reviews Microbiology, 10(1), 13–26. doi:10.1038/nrmicro2670
3. Burrows, M., & Sutton, G. (2013). Interacting gears synchronize propulsive leg movements in a jumping insect. Science, 341(6151), 1254–1256. doi:10.1126/science.1240284
4. Husnik, F., Nikoh, N., Koga, R., Ross, L., Duncan, R. P., Fujie, M., ... & McCutcheon, J. P. (2013). Horizontal gene transfer from diverse bacteria to an insect genome enables a tripartite nested mealybug symbiosis. Cell, 153(7), 1567–1578. doi:10.1016/j.cell.2013.05.040
5. Johnson, K. P., Dietrich, C. H., Friedrich, F., Beutel, R. G., Wipfler, B., Peters, R. S., ... & Yoshizawa, K. (2018). Phylogenomics and the evolution of hemipteroid insects. Proceedings of the National Academy of Sciences, 115(50), 12775–12780. doi:10.1073/pnas.1815820115
6. Boyce, G. R., Gluck-Thaler, E., Slot, J. C., Stajich, J. E., Davis, W. J., James, T. Y., ... & Kasson, M. T. (2019). Psychoactive plant- and mushroom-associated alkaloids from two behavior modifying cicada pathogens. Fungal Ecology, 41, 147–164. doi:10.1016/j.funeco.2019.06.002