STUDIES ON THE ISSECT FAUNA ON SOOTCH BROOM
187
they have been largely discuased in Sections VI and -1. Dempster
found that the spiders fed on the nymphs and adults of the broom
Miridae, and that the Orthdylw species were more heavily attacked,
aa they hatch later and their adults are in the field when the webs of
Linyphiu and the other web-epinners are larger. He found that in the
3 years 196Ck62, the number of spiders did not vary much, averaging
10 per bush each year, whereas the numbers of individual epecies varied
considerably. Thus in 1960 Xysticw was particularly common, while
Meta made up three-quarters of the web-spinners. In 1962 Phibdrmus
had replaced Xysticus as the most common thomisid and the numbers
of Metu were low.
Agwu (1967) found that Linyphia was by much the most common of
the web-spinning spiders in 1964-66, and that the adults of Lewqtera
were preyed on to a considerable extent. The numbers of Leucopteru
adults declined from 6 514 000 in 1964 to 2 710 OOO in 1965 and to
89 000 in 1966. Simultaneously, the numbera of webs on broom also
declined from 60000 in 1964, to 40000 in 1965 and 18000 in 1966.
Whether the decline of the web-spinners is causally connected with
decline of this prey remains unknown.
2. Birds
The effects of predation by birds on populations of broom aphids
(B. D. Smith, 1966a) and on those of the scolytid beetles (M. R. Smith,
1958) have been pointed out in Sections V and VI. B. D. Smith came to
the conclusion that predation by the most constant species in the area,
i.e. by Parw meruleus (Blue Tit) and by Fringilla coelebs gewferi
(Chaffinch) was one of the two most important factors in checking the
population of Acyrthosiphon spartii a t its peak. Close range observations
showed that alatae were preferentially eaten by the Blue Tita and then
the largest aphids, thua reducing the reproductive rate of the aphid
population immediately.
M. R. Smith found that the Blue Tits fed on the scolytids within the
stem mines in autumn and winter, when most of the surface living
insects had disappeared. Changes in the feeding habits of Blue Tits
have been described by Lack (1954). During August, the Tits fed more
or l e ~ s independently of each other, but in September they began to
collect to feed in groups. They frequently displaced each other from a
&em, although there was no shortage of food. By the end of the winter,
densely attacked stems were liable to be completely stripped of beetles,
whereas on stems with fewer galleries, some beetles survived at the
periphery of the gallery systems. Smith’s analysis showed that there
waa a direct relationship between the density of scolytids and the percentage attacked and the curve depicting this functional response by
187
they have been largely discuased in Sections VI and -1. Dempster
found that the spiders fed on the nymphs and adults of the broom
Miridae, and that the Orthdylw species were more heavily attacked,
aa they hatch later and their adults are in the field when the webs of
Linyphiu and the other web-epinners are larger. He found that in the
3 years 196Ck62, the number of spiders did not vary much, averaging
10 per bush each year, whereas the numbers of individual epecies varied
considerably. Thus in 1960 Xysticw was particularly common, while
Meta made up three-quarters of the web-spinners. In 1962 Phibdrmus
had replaced Xysticus as the most common thomisid and the numbers
of Metu were low.
Agwu (1967) found that Linyphia was by much the most common of
the web-spinning spiders in 1964-66, and that the adults of Lewqtera
were preyed on to a considerable extent. The numbers of Leucopteru
adults declined from 6 514 000 in 1964 to 2 710 OOO in 1965 and to
89 000 in 1966. Simultaneously, the numbera of webs on broom also
declined from 60000 in 1964, to 40000 in 1965 and 18000 in 1966.
Whether the decline of the web-spinners is causally connected with
decline of this prey remains unknown.
2. Birds
The effects of predation by birds on populations of broom aphids
(B. D. Smith, 1966a) and on those of the scolytid beetles (M. R. Smith,
1958) have been pointed out in Sections V and VI. B. D. Smith came to
the conclusion that predation by the most constant species in the area,
i.e. by Parw meruleus (Blue Tit) and by Fringilla coelebs gewferi
(Chaffinch) was one of the two most important factors in checking the
population of Acyrthosiphon spartii a t its peak. Close range observations
showed that alatae were preferentially eaten by the Blue Tita and then
the largest aphids, thua reducing the reproductive rate of the aphid
population immediately.
M. R. Smith found that the Blue Tits fed on the scolytids within the
stem mines in autumn and winter, when most of the surface living
insects had disappeared. Changes in the feeding habits of Blue Tits
have been described by Lack (1954). During August, the Tits fed more
or l e ~ s independently of each other, but in September they began to
collect to feed in groups. They frequently displaced each other from a
&em, although there was no shortage of food. By the end of the winter,
densely attacked stems were liable to be completely stripped of beetles,
whereas on stems with fewer galleries, some beetles survived at the
periphery of the gallery systems. Smith’s analysis showed that there
waa a direct relationship between the density of scolytids and the percentage attacked and the curve depicting this functional response by