Many unusual features of scale insects (Hemiptera: Coccoidea) can be explained as historical legacy. Developmental specializations in ancestral coccoids resulted in a neotenous adult female and a drastic metamorphosis of the male. Subsequent evolution led to numerous, often convergently derived, adaptations to parasitic life on higher plants. The sedentary lifestyle of female scale insects has favored the evolution of appendage reduction or loss, gross changes in body shape, and protective wax secretions, tests, and other scale covers. Morphological peculiarities of adult males relate to flight or to mating with concealed females. Scale insects have diverse egg-protecting methods, a range of chromosome behaviors (including several methods of sex determination), marked sexual dimorphism [even sometimes in first-instar nymphs (crawlers)], and more rarely sexual dichronism. Crawlers have evolved as the main agents of dispersal. The biotic interactions of scale insects include diverse endosymbioses with microorganisms, sometimes morphological and behavioral adaptations for obligate association with ants, and often highly specific host-plant associations that may lead to demic adaptation or the evolution of complex galls.

Adaptations in scale insects

Gall-inducing insects – Nature's most sophisticated herbivores

https://ucanr.edu/sites/entomology/files/149982.pdf

A preliminary phylogeny of the scale insects (Hemiptera : Sternorrhyncha : Coccoidea) based on nuclear small-subunit ribosomal DNA

Many hemipteroids are major pests and vectors of microbial pathogens, infecting crops. Saliva of the hemipteroids is critical in enabling them to be voracious feeders on plants, including the economically important ones. A plethora of hemipteroid salivary enzymes is known to inflict stress in plants, either by degrading the plant tissue or by affecting their normal metabolism. Hemipteroids utilize one of the following three strategies of feeding behaviour: salivary sheath feeding, osmotic-pump feeding and cell-rupture feeding. The last strategy also includes several different tactics such as lacerate-and-flush, lacerate-and-sip and macerate-and-flush. Understanding hemipteroid feeding mechanisms is critical, since feeding behaviour directs salivary composition. Saliva of the Heteroptera that are specialized as fruit and seed feeders, includes cell-degrading enzymes, auchenorrhynchan salivary composition also predominantly consists of cell-degrading enzymes such as amylase and protease, whereas that of the Sternorhyncha includes a variety of allelochemical-detoxifying enzymes. Little is known about the salivary composition of the Thysanoptera. Cell-degrading proteins such as amylase, pectinase, cellulase and pectinesterase enable stylet entry into the plant tissue. In contrast, enzymes such as glutathione peroxidase, laccase and trehalase detoxify plant chemicals, enabling the circumvention of plant-defence mechanisms. Salivary enzymes such as M1-zinc metalloprotease and CLIP-domain serine protease as in Acyrthosiphon pisum (Aphididae), and non-enzymatic proteins such as apolipophorin, ficolin-3-like protein and 'lava-lamp' protein as in Diuraphis noxia (Aphididae) have the capacity to alter host-plant-defence mechanisms. A majority of the hemipteroids feed on phloem, hence Ca++-binding proteins such as C002 protein, calreticulin-like isoform 1 and calmodulin (critical for preventing sieve-plate occlusion) are increasingly being recognized in hemipteroid-plant interactions. Determination of a staggering variety of proteins shows the complexity of hemipteroid saliva: effector proteins localized in hemipteran saliva suggest a similarity to the physiology of pathogen-plant interactions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0007485313000618

Salivary proteins of plant-feeding hemipteroids - implication in phytophagy.

The Thysanoptera and sternorrhynchous Hemiptera induce galls through feeding action, a behavior similar to that in the Cecidomyiidae. Salivary glands of gall-founding female thrips include greater quantities of hydrolyzing enzymes and soluble proteins than those in either males or pupae, which possibly alter the host-tissue metabolism, enabling galls to develop. Piercing-and-sucking mouthparts of the Hemiptera are adapted for an exclusively liquid diet – either the plant sap from vascular elements or the fluids from living nonvascular cells. Hemipteran-salivary chemistry alters the hormonal balance in the host, triggering gall development. Soluble proteins in the saliva of nymphs are critical. Gall-inducing Hemiptera vigorously take oxygen from the ‘gall’ tissue, which triggers auxin activity. Gall-inducing behaviors in the Thysanoptera and Sternorrhyncha are similar in that gall induction occurs by the feeding action of a single female, and gall-founding females disperse over short distances seeking youn...

https://www.tandfonline.com/doi/pdf/10.1080/17429145.2011.630847

Gall induction by hemipteroid insects

RAMAN, A. and TAKAGI, S. 1992. Galls induced on Hopea ponga (Dipterocarpaceae) in south· ern India and the gall·maker belonging to the Beesoniidae (Homoptera: Coccoidea). Ins. matsum. n. s. 47: 1-32, 36 figs. Spherical echinate galls are induced on Hopea ponga by Mangalorea hopeae Takagi, gen. et sp. nov., in southern India. Galls arise in leafaxils through modification of the vegetative bud which develops into a massive columella: differentiation and development of leaf primordia are com· pletely arrested, but the surface initials of the columella develop into multicellular echinate appendages. Each mature gall contains only one female, whereas a number of male insects occur on their maternal gall, occupying spaces between echinate appendages. M. hopeae is unusual in lacking caudal setae in the 1st instar, which is unique among the known beesoniids also in having acorn·shaped setae; the adult female is not enclosed by the skin of the preceding larval instar and is characterized by having 'warts' or much modified setae; the adult male is peculiar in having numerous papillae over the body. Authors' addresses. RAMAN, A.: Entomology Research Institute, Loyola College, Madras 600 034, India. TAKAGI, S.: Systematic Entomology, Faculty of Agriculture, Hokkaid6 University, Kita 9 Nisi 9, Kita-ku, Sapporo, 060 Japan.

/pdf/galls-induced-on-hopea-ponga-dipterocarpaceae-in-southern-2hwjab30xx.pdf

Galls induced on hopea ponga (dipterocarpaceae) in southern india and the gall-maker belonging to the beesoniidae (homoptera : coccoidea)

RAMAN, A., SINGH, R.N., and MARYANSKA·NADACHOWSKA, A. 1997. Biology and karyology of a cecidogenous psylloid, Trioza fletcheri minor (Homoptera: Psylloidea) and morphogenesis of galls on the leaves of Terminalia tomentosa and T. arjuna (Combretaceae). Ins. matsum. n. s. 53 : 117-134, Stabs., 26 figs. A subtropical plant louse, Trioza fletcheri minor, induces leaf galls on at least five species of Terminalia in the Indian subcontinent. Taking Terminalia tomentosa and T. arjuna, this paper describes cecidogenetic interactions between them and the psylloid. T. f minor completes its life cycle in c. 34 days, and the cecidogenetic process lasts 20-22 days, correlating with the develop· mental phases of the nymphal instars. Nymphal I instars initiate galls by settling on stomatal apertures of the host leaf, and the host plant responds by producing a covering growth enclosing the nymphal instars. Karyological study of the triozids raised independently from galls shows that the same species induces galls on both tomentosa and arjuna. In spite of that, probably due to genetic and physiological variation in host plants, subtle differences in the expression of mature gall form and production of trichomatous covering growth during early stages of cecidogenesis are evident. Largely, morphogenetic and metabolic responses within growing gall systems are identi· cal. Shorter egg pedicels of T. f minor appear to be an evolutionary adaptation to the host plants of mesic distribution. Authors' addresses. RAMAN, A.: Loyola College, Madras 600 034, India. SINGH, R.N.: Cen· tral Tasar Research and Training Institute, Ranchi, India. MARYANSKA·NADACHOWSKA, A.: Polish Academy of Sciences, 31-016 Krakow, Poland. Address for Correspondence: Professor A. Raman, 3/40 Cunningham Street, Dubbo, NSW 2830, Australia

/pdf/biology-and-karyology-of-a-cecidogenous-psylloid-trioza-2pjfcvkflu.pdf

Biology and karyology of a cecidogenous psylloid, trioza fletcheri minor (homoptera : psylloidea) and morphogenesis of galls on the leaves of terminalia tomentosa and t. arjuna (combretaceae)

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Galls induced on hopea ponga (dipterocarpaceae) in southern india and the gall-maker belonging to the beesoniidae (homoptera : coccoidea)

Biology and karyology of a cecidogenous psylloid, trioza fletcheri minor (homoptera : psylloidea) and morphogenesis of galls on the leaves of terminalia tomentosa and t. arjuna (combretaceae)