9. Origin and evolution of brochosomes and related behaviors
 
Apparently, both the secretory specializations of the Malpighian tubules and the behaviors in which the tubule products were applied to the integument have already been present in the common ancestor of modern Hemiptera Cicadomorpha: leafhoppers + treehoppers (Membracoidea), spittlebugs (Cercopoidea), and cicadas (Cicadoidea). In the last two groups, these specializations are only present in the immatures, playing important roles in their peculiar life styles.
 
 
 
 
Right : Hypothesis of relationships between the major lineages of Hemiptera Cicadomorpha. The numbers indicate hypothetical stages in the evolution of the Malpighian tubule function and related traits. See below for explanation:
1 - Ancestor of all Cicadomorpha. Distal parts of the Malpighian tubules were modified for synthesis of proteins, apparently only in the immatures (subterraneous?). The function of these hypothetical products is unknown.
 
2 - Ancestor of cicadas & spittlebugs. Immatures lived concealed. The distal parts of the immature Malpighian tubules manufactured proteins, and proximal parts mucopolysaccharids. The secretion was mixed into excrement. The specialization disappeared at the adult stage.
 
3 - Cicadas. Immatures live under the ground. The Malpighian tubule secreta are used to strengthen the walls of burrows and possibly also to clean the integument. The anti-fungal function is possible. The specialization disappears at the adult stage.
 
Right : A 2nd instar nymph of the periodical cicada, Magicicada sp.
 
 
4 - Spittlebugs. Immatures live within their foamed excrement or excrement-filled calcareous dwelling tubes. The Malpighian tubule secreta are used as surfactants stabilizing the foam or as a structural component of the dwelling tubes. The specialization disappears at the adult stage.
 
Right : A spittlebug nymph (Philaenus) concealed within a mass of its foamed excretion, also known as "spittle" or "cuckoo-spit".
 
5 - Leafhoppers. The Malpighian tubules are active as glands in both the immatures and adults. The main secretory product, brochosomes, are released separately from excrement and applied to the integument in anointing and grooming behaviors. Legs are armed with rows and groups of strong setae manipulating the particles. Hypothetical function: water- and excrement-repellence, various other kinds of protection.
 
6 - Leafhoppers (part). The Malpighian tubules of immatures produce non-brochosomal alternative products switching to production of brochosomes during the last nymphal instar. Immatures display a peculiar anointing behavior, "bathing".
 
7 - Treehoppers. Brochosomes lost. The Malpighian tubules produce non-brochosomal alternative secreta in both the immatures and adults. Anointing retained. Immatures display "bathing".
Research questions
Are brochosomes homologous to the proteinaceous secreta of the cicada and spittlebug immatures? -- Unfortunately, the chemical nature of the tubule secretory products is not yet known in enough detail to prove that the same ancestral secretory mechanisms are used in both cases.
Did brochosomes evolve from products with a less complex structure? Did their peculiar geometric structure evolve gradually? -- Because all the extant leafhoppers have basically similar brochosomes, nothing is known about the pre-brochosomic period of the leafhopper evolution.
Read more: Rakitov 2002a, b
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