Dyar's Law (or "Dyar's Rule") is the observed standard that during development of the immature stages of an

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

, increases in highly

sclerotized Sclerosis (also sclerosus in the Latin names of a few disorders) is a hardening of tissue and other anatomical features. It may refer to: * Sclerosis (medicine), a hardening of tissue * in zoology, a process which forms sclerites, a hardened exo ...

body parts are predictable and regular by a relatively constant factor. The law is named after Harrison G. Dyar who in 1890 published a paper about his observation that the head capsule widths of

lepidopteran Lepidoptera ( ) or lepidopterans is an order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organisms, making it ...

larvae follow a

geometric progression A geometric progression, also known as a geometric sequence, is a mathematical sequence of non-zero numbers where each term after the first is found by multiplying the previous one by a fixed number called the ''common ratio''. For example, the s ...

in growth. However, various authors have noted that an earlier publication in 1886 by W.K. Brooks independently described the same phenomenon in

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s, and therefore the variant term Brooks-Dyar Law (or "Brooks Rule" or "Brooks-Dyar Rule") also commonly appears in the literature. The earliest known citation of either author's observations constituting a "Law" dates to a 1925 reference to "Dyar's Law". Though the progression can be influenced by

abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them und ...

and biotic factors such as temperature and food, Dyar's Law can be accurately used to differentiate

instars An instar (, from the Latin ''wikt:instar#Latin, īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each ecdysis, moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the ...

of immature insects and to predict the size of instars missing from samples, crucial data for accurately delineating insect developmental histories. Initially based on observations of crustaceans and insect larvae, this Law has been applied to immature arthropods in general. Some 80% of entomological studies published from 1980 to 2007 that examined the validity of Dyar's observations supported the Law.

References

{{reflist Arthropod anatomy