python - 为什么用requests库能爬取而用scrapy却不能?

# -*- coding: utf-8 -*-
import requests


def xici_request():
    url = 'http://www.xicidaili.com'
    headers = {
        'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8',
        'Accept-Encoding': 'gzip, deflate, sdch',
        'Accept-Language': 'zh-CN,zh;q=0.8',
        'Cache-Control': 'max-age=0',
        'Connection': 'keep-alive',
        'Host': 'www.xicidaili.com',
        'Referer': 'https://www.google.com/',
        'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36'
   
    res = requests.get(url, headers=headers)
    print(res.text)

if __name__ == '__main__':
    xici_request()

# -*- coding: utf-8 -*-
import scrapy
from collectips.items import CollectipsItem


class XiciSpider(scrapy.Spider):
    name = "xici"
    allowed_domains = ["http://www.xicidaili.com"]

    headers = {'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8',
               'Accept-Encoding': 'gzip, deflate, sdch',
               'Accept-Language': 'zh-CN,zh;q=0.8',
               'Cache-Control': 'max-age=0',
               'Connection': 'keep-alive',
               'Host': 'www.xicidaili.com',
               'Referer': 'https://www.google.com/',
               'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36'}

    def start_requests(self):
        reqs = []
        for i in range(1, 21):
            req = scrapy.Request(
                'http://www.xicidaili.com/nn/{}'.format(i), headers=self.headers)
            reqs.append(req)

        return reqs

    def parse(self, response):
        item = CollectipsItem()
        sel = response.selector
        for i in range(2, 102):
            item['IP'] = sel.xpath(
                '//*[@id="ip_list"]/tbody/tr[{}]/td[2]/text()'.format(i)).extract()
            item['PORT'] = sel.xpath(
                '//*[@id="ip_list"]/tbody/tr[{}]/td[3]/text()'.format(i)).extract()
            item['DNS_POSITION'] = sel.xpath(
                '//*[@id="ip_list"]/tbody/tr[{}]/td[4]/a/text()'.format(i)).extract()
            item['TYPE'] = sel.xpath(
                '//*[@id="ip_list"]/tbody/tr[{}]/td[6]/text()'.format(i)).extract()
            item['SPEED'] = sel.xpath(
                '//*[@id="ip_list"]/tbody/tr[{}]/td[7]/p[@title]'.format(i)).extract()
            item['LAST_CHECK_TIME'] = sel.xpath(
                '//*[@id="ip_list"]/tbody/tr[{}]/td[10]/text()'.format(i)).extract()
            yield item

代码如上,为什么requests能返回网页内容,而scrapy却是报错内部服务器错误500? 请大神解救??