379
realm that encompasses terrestrial and freshwater ecosystems where known cases of
parasitism expand dramatically. The parasitoid guild in continental ecosystems is
represented by at least 84 separate originations among 10 orders of insects, amounting to 10% of all described insect species (Gaston 1991; Weinstein and Kuris 2016).
Considering only parasitoids, rather than the broader category that would include
inquilines, parasites and cleptoparasitoids, there are 68,000 out of 850,000 described
insect species, or about 8% of the total, that are obligately parasitoid species (Gaston
1991; Godfray 1994). The majority of insect parasitoids are typically small, inconspicuous wasps of Hymenoptera (sawflies, wasps, ants and bees), constituting about
75% of the total, and Diptera (true flies) account for another 20% (Eggleton and
Belshaw 1992). The remaining 5% is scattered among lineages of Neuroptera (lacewings and antlions), Coleoptera (beetles), Strepsiptera (twisted-wing parasites),
Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) (Askew 1971;
Eggleton and Belshaw 1992; Godfray 1994). However, because of their inconspicuousness, rarity and frequent fidelity to a single host, there is significant underreporting of parasitoid taxa in local, community-level assessments. This
under-representation especially is true for apocritan wasps (LaSalle and Gauld
1991) and tachinid Diptera (Crosskey 1980; also see Stireman III 2005). Given
these and other recent updates and trends in species descriptions, it is probable that
up to 25% of insect species have parasite or parasitoid life habits (Godfray 1994),
and that 20% of insect species are solely parasitoids (Hochberg and Hawkins 1992).
Historically, a major issue regarding macroevolutionary patterns in parasite and
especially parasitoid insects has been whether diet specialization, as opposed to
consumption of an eclectic spectrum of food, has been a pathway for increased
diversification (Rainford and Mayhew 2015). One early test of these two contending
hypotheses was sister clade comparisons of lineages in which one clade possessed
a non-parasitoid diet and its sister clade engaged in an exclusively parasitoid diet
(Wiegmann et al. 1993). The question posed by the authors of that study was
whether the parasitoid life habit (see below for a definition) was an evolutionary
dead end involving specialization, or rather a key innovation that led to expansive
speciation (Wiegmann et al. 1993). The authors found, of 15 clades examined, that
6 were significantly more diverse and 9 less diverse than their saprophage or predator sister clades, leading to the conclusion that a parasitic or parasitoid life style
does not result in increased diversification rates and may very well be an evolutionary dead end. However, the “push of the past” phenomenon, whereby clades that
persist for a significant length of deep time typically experience high levels of early
diversification but subsequently have substantially decreased levels (Budd and
Mann 2018), is a relevant consideration. The push-of-the-past phenomenon may
indicate that currently depauperate parasitoid clades have experienced elevated speciation levels in the past. Nevertheless, this seeming paradox of elevated specialization and evolution of a highly restricted life habit yielding a dramatic increase in
speciosity (Drake 2003) was retested by Rainford and Mayhew (2015) by employing a different approach, methodology and analyses (details are provided in Rainford
and Mayhew 2015). The results of Rainford and Mayhew (2015) indicated that
there are well-characterized bouts of diversification of parasitoidism involving parasitoid wasps (Rainford and Mayhew 2015), where the bulk of parasitoidism,
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution
realm that encompasses terrestrial and freshwater ecosystems where known cases of
parasitism expand dramatically. The parasitoid guild in continental ecosystems is
represented by at least 84 separate originations among 10 orders of insects, amounting to 10% of all described insect species (Gaston 1991; Weinstein and Kuris 2016).
Considering only parasitoids, rather than the broader category that would include
inquilines, parasites and cleptoparasitoids, there are 68,000 out of 850,000 described
insect species, or about 8% of the total, that are obligately parasitoid species (Gaston
1991; Godfray 1994). The majority of insect parasitoids are typically small, inconspicuous wasps of Hymenoptera (sawflies, wasps, ants and bees), constituting about
75% of the total, and Diptera (true flies) account for another 20% (Eggleton and
Belshaw 1992). The remaining 5% is scattered among lineages of Neuroptera (lacewings and antlions), Coleoptera (beetles), Strepsiptera (twisted-wing parasites),
Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) (Askew 1971;
Eggleton and Belshaw 1992; Godfray 1994). However, because of their inconspicuousness, rarity and frequent fidelity to a single host, there is significant underreporting of parasitoid taxa in local, community-level assessments. This
under-representation especially is true for apocritan wasps (LaSalle and Gauld
1991) and tachinid Diptera (Crosskey 1980; also see Stireman III 2005). Given
these and other recent updates and trends in species descriptions, it is probable that
up to 25% of insect species have parasite or parasitoid life habits (Godfray 1994),
and that 20% of insect species are solely parasitoids (Hochberg and Hawkins 1992).
Historically, a major issue regarding macroevolutionary patterns in parasite and
especially parasitoid insects has been whether diet specialization, as opposed to
consumption of an eclectic spectrum of food, has been a pathway for increased
diversification (Rainford and Mayhew 2015). One early test of these two contending
hypotheses was sister clade comparisons of lineages in which one clade possessed
a non-parasitoid diet and its sister clade engaged in an exclusively parasitoid diet
(Wiegmann et al. 1993). The question posed by the authors of that study was
whether the parasitoid life habit (see below for a definition) was an evolutionary
dead end involving specialization, or rather a key innovation that led to expansive
speciation (Wiegmann et al. 1993). The authors found, of 15 clades examined, that
6 were significantly more diverse and 9 less diverse than their saprophage or predator sister clades, leading to the conclusion that a parasitic or parasitoid life style
does not result in increased diversification rates and may very well be an evolutionary dead end. However, the “push of the past” phenomenon, whereby clades that
persist for a significant length of deep time typically experience high levels of early
diversification but subsequently have substantially decreased levels (Budd and
Mann 2018), is a relevant consideration. The push-of-the-past phenomenon may
indicate that currently depauperate parasitoid clades have experienced elevated speciation levels in the past. Nevertheless, this seeming paradox of elevated specialization and evolution of a highly restricted life habit yielding a dramatic increase in
speciosity (Drake 2003) was retested by Rainford and Mayhew (2015) by employing a different approach, methodology and analyses (details are provided in Rainford
and Mayhew 2015). The results of Rainford and Mayhew (2015) indicated that
there are well-characterized bouts of diversification of parasitoidism involving parasitoid wasps (Rainford and Mayhew 2015), where the bulk of parasitoidism,
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution